Mini Alarm

Suitable for doors & windows, Portable anti-bag-snatching unit

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This circuit, enclosed in a small plastic box, can be placed into a bag or handbag. A small magnet is placed close to the reed switch and connected to the hand or the clothes of the person carrying the bag by means of a tiny cord. If the bag is snatched abruptly, the magnet looses its contact with the reed switch, SW1 opens, the circuit starts oscillating and the loudspeaker emits a loud alarm sound. A complementary transistor-pair is wired as a high efficiency oscillator, directly driving a small loudspeaker. Low part-count and 3V battery supply allow a very compact construction.

Parts:

R1 = 330K
R2 = 100R
C1 = 10nF-63V
C2 = 100uF-25V
Q1 = BC547
Q2 = BC327
B1 = 3V Battery or Two AA Cells in Series
SW1 = Read Switch & Small Magnet
SPKR = 8R Loudspeaker (See Notes)

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Notes:

• The loudspeaker can be any type; its dimensions are limited only by the box that will enclose it.
• An on-off switch is unnecessary because the stand-by current drawing is less than 20µA.
• Current consumption when the alarm is sounding is about 100mA.
• If the circuit is used as anti-bag-snatching, SW1 can be replaced by a 3.5mm mono Jack socket and the magnet by a 3.5mm. Mono Jack plugs having its internal leads shorted. The Jack plug will be connected to the tiny cord etc.
• Do not supply this circuit at voltages exceeding 4.5V: it will not work and Q2 could be damaged. In any case a 3V supply is the best compromise.

Two-Tone Siren Using One IC

This circuit is intended for children fun, and can be installed on bicycles, battery powered cars and motorcycles, but also on models and various games and toys. With SW1 positioned as shown in the circuit diagram, the typical dual-tone sound of Police or Fire-brigade cars is generated, by the oscillation of IC1A and IC1B gates. With SW1 set to the other position, the old siren sound increasing in frequency and then slowly decreasing is reproduced, by pushing on P1 that starts oscillation in IC1C and IC1D.

The loudspeaker, driven by Q1, should be of reasonable dimensions and well encased, in order to obtain a more realistic and louder output. Tone and period of the sound oscillations can be varied by changing the values of C1, C2, C5, C6 and/or associated resistors. No power switch is required: leave SW1 in the low position (old-type siren) and the circuit consumption will be negligible.

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One IC Two-Toness Siren Circuit Diagram

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Parts:

R1 = 470K - 1/4W Resistors
R2 = 680K - 1/4W Resistor
R3 = 470K - 1/4W Resistors
R4 = 82K - 1/4W Resistor
R5 = 330K - 1/4W Resistor
R6 = 10K - 1/4W Resistor
R7 = 33K - 1/4W Resistor
R8 = 3.3M - 1/4W Resistor

C1 = 10µF - 25V Electrolytic Capacitors
C2 = 10nF - 63V Polyester Capacitors
C3 = 100nF - 63V Polyester Capacitor
C4 = 100µF - 25V Electrolytic Capacitor
C5 = 10µF - 25V Electrolytic Capacitors
C6 = 10nF - 63V Polyester Capacitors

D1 = 1N4148 - 75V 150mA Diodes
D2 = 1N4148 - 75V 150mA Diodes
D3 = 1N4148 - 75V 150mA Diodes

Q1 = BC337 - 45V 800mA NPN Transistor
P1 = SPST Pushbutton
B1 = 6V Battery (4 AA 1.5V Cells in series)
IC1 = 4093 - Quad 2 input Schmitt NAND Gate IC
SW1 = DPDT Switch
SPKR= 8 Ohm Loudspeaker

Guitar Control

Stand-alone, 9V battery powered unit, Three-level input selector, three-band tone control

This preamplifier was designed as a stand-alone portable unit, useful to control the signals generated by guitar pick-ups, particularly the contact "bug" types applied to acoustic instruments. Obviously it can be used with any type of instrument and pick-up. It features a -10dB, 0dB and +10dB pre-set input selector to adjust input sensitivity, in order to cope with almost any pick-up type and model. A very long battery life is ensured by the incredibly low current consumption of this circuit, i.e. less than 800µA.

Parts:

P1,P2_________100K Linear Potentiometers
P3____________470K Linear Potentiometer
P4_____________10K Log. Potentiometer
R1____________150K 1/4W Resistor
R2____________220K 1/4W Resistor
R3_____________56K 1/4W Resistor
R4____________470K 1/4W Resistor
R5,R6,R7_______12K 1/4W Resistors
R8,R9___________3K9 1/4W Resistors
R10,R11_________1K8 1/4W Resistors
R12,R13________22K 1/4W Resistors
C1____________220nF 63V Polyester Capacitor
C2,C8___________4µ7 63V Electrolytic Capacitors
C3_____________47nF 63V Polyester Capacitor
C4,C6___________4n7 63V Polyester Capacitors
C5_____________22nF 63V Polyester Capacitor
C7,C9_________100µF 25V Electrolytic Capacitors
IC1___________TL062 Low current BIFET Dual Op-Amp
J1,J2__________6.3mm. Mono Jack sockets
SW1______________1 pole 3 ways rotary or slider switch
SW2______________SPST Switch
B1_______________9V PP3 Battery Clip for PP3 Battery

Circuit operation:

IC1A op-amp is wired as an inverting amplifier, having its gain set by a three ways switch inserting different value resistors in parallel to R4. This input stage is followed by an active three-band tone control stage having unity gain when controls are set in their center position and built around IC1B.

Technical data:

Frequency response:
20Hz to 20KHz -0.5dB, controls flat.
Tone control frequency range:
±15dB @ 30Hz; ±19dB @ 1KHz; ±16dB @ 10KHz.
Maximum input voltage (controls flat):
900mV RMS @ +10dB input gain; 7.5V RMS @ -10dB input gain.
Maximum undistorted output voltage:
2.5V RMS.
Total Harmonic Distortion measured @ 2V RMS output:
<0.012% @ 1KHz; <0.03% @ 10KHz.
THD @ 1V RMS output:
less 0.01%
Total current drawing:
less 800µA.

Heart Rate Monitor

Strictly speaking, this simple circuit shouldn't work! How could anyone expect an ordinary light dependent resistor photo cell to 'see' through a fingertip in natural daylight and detect the change in blood flow as the heart pulsates? The secret is a high gain circuit, based on a dual op amp IC which can be either the low power LM358 or the JFET TL072. The LDR is connected in series across the 9V battery supply via a 100kO resistor (R1) and the minute signal caused by the blood pulsing under the skin is fed to the non-inverting (+) input, pin 3, of IC1a via a 0.µF capacitor.

Pin 3 is biased by a high impedance voltage divider consisting of two 3.3MO resistors. The feedback resistors to pin 2 set the gain to 11 times. The output of IC1a is fed via a 0.47µF capacitor and 220kO resistor to IC1b. This is configured as an inverting op amp with a gain of 45 so that the total circuit gain is about 500. The output of IC1b is used to drive an analog meter which may be a multimeter set to the 10V DC range or any panel meter in series with a resistor to limit the current to less than its full-scale deflection. The prototype used an old VU meter with a 47kO resistor fitted in series.

 

 

Note that the unit was designed to use the Dick Smith Electronics light dependent resistor (Z-4801). Other LDRs may require a change in the value of resistor R1. A light source such as a high brightness LED is not required. All that is needed is a reasonably well-lit room, preferably natural daylight, to produce a healthy swing of the needle. Only when the hands are very cold does it make it a little more difficult to accurately count the pulses. To check your heart rate, carefully position your thumb or finger over the LDR and count the meter fluctuations for a period of 15 seconds. Then multiply the result by four to obtain your pulse rate. The circuit can not be used if you are walking or running, etc.

Digital Step-Km Counter Circuit Schematic

This circuit measures the distance covered during a walk. Hardware is located in a small box slipped in pants' pocket and the display is conceived in the following manner: the leftmost display D2 (the most significant digit) shows 0 to 9 Km. and its dot is always on to separate Km. from hm. The rightmost display D1 (the least significant digit) shows hundreds meters and its dot illuminates after every 50 meters of walking.

A beeper (excludable), signals each count unit, occurring every two steps. A normal step was calculated to span around 78 centimeters, thus the LED signaling 50 meters illuminates after 64 steps (or 32 operations of the mercury switch), the display indicates 100 meters after 128 steps and so on.

For low battery consumption the display illuminates only on request, pushing on P2. Accidental reset of the counters is avoided because to reset the circuit both pushbuttons must be operated together. Obviously, this is not a precision meter, but its approximation degree was found good for this kind of device. In any case, the most critical thing to do is the correct placement of the mercury switch inside of the box and the setting of its sloping degree.

Parts:

R1 = 22K 1/4W Resistor
R2 = 2.2M 1/4W Resistor
R3 = 22K 1/4W Resistor
R4 = 1M 1/4W Resistor
R5 = 4.7K 1/4W Resistor
R6 = 47R 1/4W Resistor
R7 = 4.7K 1/4W Resistor
R8 = 4.7K 1/4W Resistor
R9 = 1K 1/4W Resistor

C1 = 47nF 63V Polyester Capacitor
C2 = 100nF 63V Polyester Capacitor
C3 = 10nF 63V Polyester Capacitor
C4 = 10µF 25V Electrolytic Capacitor

D1 = Common-cathode 7-segment LED mini-display (Hundreds meters)
D2 = Common-cathode 7-segment LED mini-display (Kilometers)

Q1 = BC327 45V 800mA PNP Transistors
Q2 = BC327 45V 800mA PNP Transistors

P1 = SPST Pushbutton (Reset)
P2 = SPST Pushbutton (Display)

IC1 = 4093 Quad 2 input Schmitt NAND Gate IC
IC2 = 4024 7 stage ripple counter IC
IC3 = 4026 Decade counter with decoded 7-segment display outputs IC
IC4 = 4026 Decade counter with decoded 7-segment display outputs IC

SW1 = SPST Mercury Switch, called also Tilt Switch
SW2 = SPST Slider Switch (Sound on-off)
SW3 = SPST Slider Switch (Power on-off)

BZ = Piezo sounder
B1 = 3V Battery (2 AA 1.5V Cells in series)

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Circuit operation:

IC 1A & IC 1B form a monostable multi vibrator providing some degree of freedom from excessive bouncing of the mercury switch. Therefore a clean square pulse enters IC2 that divides by 64. Q2 drives the LED dot-segment of D1 every 32 pulses counted by IC2. Either IC3 & IC4 divide by 10 and drive the displays. P1 resets the counters and P2 enables the displays. IC1C generates an audio frequency square wave that is enabled for a short time at each monostable count. Q1 drives the piezo sounder and SW2 allows disabling the beep.

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Notes:

• Experiment with placement and sloping degree of mercury switch inside the box: this is very critical.
• Try to obtain a pulse every two walking steps. Listening to the beeper is extremely useful during setup.
• Trim R6 value to change beeper sound power.
• Push P1 and P2 to reset.
• This circuit is primarily intended for walking purposes. For jogging, further great care must be used with mercury switch placement to avoid undesired counts.
• When the display is disabled current consumption is negligible, therefore SW3 can be omitted.

Mosquito Repeller " pengusir Nyamuk "

With the return of the fine weather, you’ll doubtless be enjoying lazing around of an evening on your patio or in your garden, but even if you’re not surrounded by marshes or other shallow water it’s very likely some intruding mosquitoes will come along to spoil this idyllic scene. Although indoors it’s easy to get rid of them these days, indeed even to prevent them coming into the house, the same can’t be said for the great outdoors. We might mention the well-known Chinese coils – the only thing Chinese about them is undoubtedly their name – which very often drive people away as much as mosquitoes, if not more! Moreover they are nasty things to handle.

There are also UV (ultra-violet) ‘electrocutors’ consisting of a blue lamp surrounded by two closely-spaced grilles between which a high voltage is applied. The mosquitoes (and fies and other flying insects) are supposedly attracted by the color of the lamp and as they approach, get electrocuted in contact with the two grilles. The only thing you have to do is pull out the drawer from time to time and get rid of the mass of dead insects. Even though the effectiveness of these first two products remains questionable, it is less so than the one we’re nonetheless going to describe here.

We’re talking about an ultrasonic mosquito repellent. The principle, as described by its numerous promoters, is as follows. Only the female mosquitoes bite (that at least is an undisputed scientific fact) and they bite when they need to feed, and above all, to feed their eggs. In this situation, they seek to avoid the males whose ‘job’ has already been done, and so they fy away from the frequencies emitted by the males when they are on heat. This is where opinions now diverge.

According to certain publications, the frequency emitted by the male mosquitoes is said to be around 20–25 kHz, and so within the realm of ultrasound. But according to others, it is in the region of 5–7 kHz instead; frequencies that a human ear, even an elderly one, can still hear very well. Rather than spending lots of money (of the order of tens of pounds) buying such a device, which moreover generally have a fixed frequency, we’re suggesting building one yourself so that you can carry out your own research this summer, especially since the circuit proposed is very simple and cheap to build.

As the figure shows, it uses just a single IC, a CMOS type 4047. This very multi-purpose IC can be wired in very many operating modes, including that of the multivibrator or astable used here. The operating frequency is set by the external components C1, R1, and P1; the latter makes it possible to slightly adjust the frequency, given the uncertainty that exists over the most efective value…To best reproduce the high frequencies produced by the generator, the output transducer used is a simple tweeter, but it must be a piezo one. Such a tweeter behaves in fact much like a capacitor, and so doesn’t overload the CMOS IC outputs that are incapable of supplying a substantial current, as everyone knows who’s ever worked with 400 series CMOS logic.

To obtain an output signal of sufficient amplitude while being powered from a single 9 V battery, this tweeter is connected between the 4047’s Q and Q outputs, making it possible to apply complementary (antiphase) signals to the tweeter so it ‘sees’ an alternating voltage of double the supply voltage. In purely theoretical terms, this quadruples the output power available. In practice, it’s better to regard it as tripling it, but the beneft achieved by doing it this way is nonetheless very real. All that remains is for you to place the project in the middle of the patio table or beside your lounger in order to get a taste of the calm of a summer’s evening without mosquitoes bothering you acoustically or worse, biting. At any rate, that’s what we wish for you…

Fully Adjustable Power Supply

Based on a National Semiconductor application note, this circuit uses an LM317 3-terminal regulator (REG1), chosen because of its built-in over-current and over-temperature protection. Its output is boosted up to just over 5A by the MJ2955 transistor (Q1). The output voltage is varied by adjusting the voltage on REG1’s ADJ terminal using VR1 (a 10kO potentiometer), via the 270O resistor. Adjustable current limiting is provided by op amp IC1, used as a comparator, which monitors the voltage across the 0.1O current sensing resistors. Once this voltage exceeds a level set by potentiometer VR2, then its output goes low, dragging down the adjust pin of REG1 and thus the output voltage.

LED1 illuminates when current limiting is occurring. The 10kO voltage adjust potentiometer (VR1) has one side connected to -5V instead of 0V so that the output voltage can be varied down to 0V instead of 1.2V (normal limit of an LM317). Trimpot VR3 is adjusted to set the minimum output voltage to +100mV or so. Note that because the -5V rail is used as a reference, it should be regulated using an LM7905 or similar. The LM317 3-terminal regulator and Q1 should be mounted on the same heatsink to take advantage of REG1’s thermal control.

Simple Oscillator / Pipe Locator

Sometimes the need arises to construct a really simple oscillator. This could hardly be simpler than the circuit shown here, which uses just three components, and offers five separate octaves, beginning around Middle C (Stage 14). Octave # 5 is missing, due to the famous (or infamous) missing Stage 11 of the 4060B IC. We might call this a Colpitts ‘L’ oscillator, without the ‘C’. Due to the reactance of the 100-µH inductor and the propagation delay of the internal oscillator, oscillation is set up around 5 MHz. When this is divided down, Stage 14 approaches the frequency of Middle C (Middle C = 261.626 Hz). Stages 13, 12, 10, and 9 provide higher octaves, with Stages 8 to 4 being in the region of ultrasound.

Simple Oscillator/Pipe Locator Circuit Diagram

 If the oscillator’s output is taken to the aerial of a Medium Wave Radio, L1 may serve as the search coil of a Pipe Locator, with a range of about 50 mm. This is tuned by finding a suitable hetero-dyne (beat note) on the medium wave band. In that case, piezo sounder Bz1 is omitted. The Simple Oscillator / Pipe Locator draws around 7 mA from a 9-12 V DC source.

Electronic Code Lock

Nowadays, electronic code locks are usually based on microcontrollers. However, if you like your electronics discrete, you will enjoy the battery-operated circuit shown here. Since the circuit automatically switches off after the door has been opened and draws no current in the idle state, three alkaline batteries (mignon, AA or R6 cells) are good for around 5,000 door openings. The main advantage is that the door opener can also be powered from the battery, so it’s not necessary to run any extra cables.

 

Project  Image :

 

 

Figure 1 shows the schematic diagram of the circuit, which is split into two parts. The first part is the control panel, which consists of a 12-position keypad and two LEDs. The second part is the programming and evaluation logic, which contains only standard logic ICs. The control panel is connected to the logic board by a 16-way flat cable. The keypad circuit is laid out with separate connections to the individual switches, instead of a matrix. The code is programmed using the two pin connector strips K1 and K2.  The circuit allows any desired combination of numbers to be used for the code, up to a maximum of 9 positions. Press-ing a particular button, which in principle is random but which naturally must be specified in advance, awakens the circuit from the zero-current idle state. This Start button cannot be used in the subsequent code sequence. The Start button is programmed by connecting a wire bridge from the associated pin of K2 to pin 1 of K1.  The code sequence is programmed in a similar manner. The first numeral of the code is programmed by connect-ing the associated pin of K2 to pin 2 of K1, the connection for the second numeral is made to pin 3 of K1, the third to pin 4 and so on. Numerals that are not used in the code do not actually have to be connected. However, if the unused buttons are connected to VDD, the code lock will assume that an error has occurred if any of these buttons is pressed and will reset the circuit. Pressing the Start button switches on transistor T1, which connects the supply voltage source to the code lock. This is indicated by the yellow LED (D20).

 

Circuit diagram :

 

 

Part List :

Resistors :

R1,R2 = not fitted
R3 = 220kΩ
R4,R5 = 1MΩ
R6 = 220kΩ
R7,R9,R10,R17 = 100kΩ
R8,R12,R14 = 2MΩ2
R11 = 560Ω
R13,R15,R20 = 1kΩ5
R16 = 100kΩ
R18 = 120Ω
R19 = 10k
R21-R24 = 3Ω3
R25-R35 = 22kΩ
Capacitors :
C1,C6,C7,C8,C10 = 100nF
C2,C3,C5 = 10nF
C4 = 1µF
C9 = 330nF
C11 = 47µF 16V radial
Semiconductors :
D1-D9,D11,D13,D14,D15,D17,
D18 = 1N4148
D10,D12 = zener diode 1V2
0.4W*
D16 = 1N4001
D19 = LED, green
D20 = LED, yellow
T1 = BC327
T2,T3,T4 = BC337
T5 = BD140
IC1 = 4017
IC2,IC3 = 4069 or 40106
Miscellaneous :
JP1 = jumper
K1,K2 = 12-way pinheader or
wire links
K3,K4 = not required (ribbon
cable )
K5, K6 = 2-way PCB terminal
block, lead pitch 5mm
S1-S12 = pushbutton with
make contact
 

Since the logic ICs are now enabled, the output of IC3f will be High, so T2 also conducts and pulls the base of T1 to ground. This means that the Start button can be released without affecting the circuit. However, C11 can now slowly charge via the high resistance of R12 until the voltage at the inverter input is high enough to cause its output to go Low, which interrupts the supply voltage to the circuit and puts it back into the idle state. The valid code must therefore be entered during the time interval determined by this R–C time constant. Once the supply voltage is disconnected, C11 discharges rapidly via D18. This is important, since other-wise C11 could retain its charge for a long time. This would make the time allowed for entering the code significantly shorter the next time the lock is used.  Pressing the Start button also has other consequences. Via the Start switch, ground potential arrives at IC2d, where it causes a pulse to be generated that places counter IC1 in a defined state (Q0 = 1) prior to the entry of the first code numeral. The first code numeral can now be entered. If the correct button is pressed, the High potential from Q0 passes through the closed switch to reach IC2d–IC2a. This net-work generates a positive pulse at the instant that the but-ton is released. This pulse clocks the counter, so that the High level from Q0 moves by one position to Q1. This process repeats itself until all code numerals have been entered. 

 

  PCB Layout :

 

 

After the ninth numeral has been entered, the positive volt-age jumps to Q9, where it charges C4 (if jumper JP1 is installed). While C4 is charging, the output of IC2e goes Low for approximately two seconds, and the output of IC3d goes high for the same interval. Power transistor T5 is switched on via R19 and T4 to supply current to the door opener. At the same time, IC3a switches on the green LED (D19) to indicate that the door can be opened. T3 limits the current through the door opener to around 700 mA. Once C4 is sufficiently charged, the output of IC2e changes to High. Not only does this switch off the door opener, but the positive edge also generates a pulse in the network IC2f/IC3c that passes through D14 to reach IC1 as a reset pulse (D14, D17 D13 and R7 together form a ‘wired-or’ gate). Inverter IC3b also provides the power-up reset to the counter. The reset signal places the circuit back into its initial state. What happens if an incorrect button is pushed? In such a case a Low level is passed through in place of the High level from the counter output. This has the same consequence as the Low level from the Start button: the counter is reset.

 

Note that you can also modify the circuit to use fewer than nine numerals for the code. All that is necessary is to connect C4 via a jumper to another counter output in place of Q9 (for example, to Q4 for a four-position code). The diode at the selected output of the counter can be replaced by a wire jumper, and the ‘higher level’ diodes can also be omit-ted. The active ‘on’ time of the door opener is 2 s. If this seems to be too short, the value of R8 or C4 can be increased. However, this also increases the amount of power drawn from the battery, especially considering that the door opener is by far the biggest power glutton in the circuit. In order to integrate the circuit into an existing door opener or to use it to operate an ac door opener, you should connect a relay to K5. Before assembling the circuit using the printed circuit board shown in Figure 2, you should separate the two sections by sawing between K3 and K4. The logic board should not be fitted directly behind the pushbuttons for entering the code. Instead, it is better to separate the entry pushbuttons, the LEDs and the door opener from the logic circuit board with a length of cable. Otherwise, a screw driver or a bit of wire connected between the emitter and collector of T5 is all that is needed to outfox the code lock and open the door.

 

Fitting the components to the circuit board should not be difficult. The ICs can be mounted in sockets. The author used 4049 inverter ICs, but in the Elektor Electronics lab prototype we used 4069’s, which are functionally compatible but not pin-compatible, and we also tried a 40106, which has Schmitt-trigger inputs. With a 4069, normal 1N4148 diodes can be used for D10 and D12. The best solution is to use the relatively noise-immune 40106. However, it is then necessary to use Zener diodes for D10 and D12, due to the higher threshold voltage. A 3.3-V type is ideal with an operating voltage of 15 V. There is one thing you should not overlook: with low-voltage zener diodes, the band on the package marks the anode instead of the cathode, as you would normally expect. At least, this is true in most cases, but not always.

 

Capacitor Discharge Ignition Circuit (CDI)

The CDI ignition circuit produces a spark from an ignition coil by discharging a capacitor across the primary of the coil. A 2uF capacitor is charged to about 340 volts and the discharge is controlled by an SCR. A Schmitt trigger oscillator (74C14) and MOSFET (IRF510) are used to drive the low voltage side of a small (120/12 volt) power transformer and a voltage doubler arrangement is used on the high voltage side to increase the capacitor voltage to about 340 volts. A similar Schmitt trigger oscillator is used to trigger the SCR about 4 times per second. The power supply is gated off during the discharge time so that the SCR will stop conducting and return to it's blocking state. The diode connected from the 3904 to pin 9 of the 74C14 causes the power supply oscillator to stop during discharge time. The circuit draws only about 200 milliamps from a 12 volt source and delivers almost twice the normal energy of a conventional ignition circuit. High voltage from the coil is about 10KV using a 3/8 inch spark gap at normal air temperature and pressure. Spark rate can be increased to possibly 10 Hertz without losing much spark intensity, but is limited by the low frequency power transformer and duty cycle of the oscillator. For faster spark rates, a higher frequency and lower impedance supply would be required. Note that the ignition coil is not grounded and presents a shock hazard on all of it's terminals. Use CAUTION when operating the circuit. An alternate method of connecting the coil is to ground the (-) terminal and relocate the capacitor between the cathode of the rectifier diode and the positive coil terminal. The SCR is then placed between ground and the +340 volt side of the capacitor. This reduces the shock hazard and is the usual configuration in automotive

555 Tone Generator (8 ohm speaker)

This is a basic 555 squarewave oscillator used to produce a 1 Khz tone from an 8 ohm speaker. In the circuit on the left, the speaker is isolated from the oscillator by the NPN medium power transistor which also provides more current than can be obtained directly from the 555 (limit = 200 mA). A small capacitor is used at the transistor base to slow the switching times which reduces the inductive voltage produced by the speaker. Frequency is about 1.44/(R1 + 2*R2)C where R1 (1K) is much smaller than R2 (6.2K) to produce a near squarewave.

Lower frequencies can be obtained by increasing the 6.2K value, higher frequencies will probably require a smaller capacitor as R1 cannot be reduced much below 1K. Lower volume levels can be obtained by adding a small resistor in series with the speaker (10-100 ohms). In the circuit on the right, the speaker is directly driven from the 555 timer output. The series capacitor (100 uF) increases the output by supplying an AC current to the speaker and driving it in both directions rather than just a pulsating DC current which would be the case without the capacitor. The 51 ohm resistor limits the current to less than 200 mA to prevent overloading the timer output at 9 volts. At 4.5 volts, a smaller resistor can be used.

Game Show Indicator Lights (Who's First)

Parts List:

Quantity       Description                   Radio Shack Part Number

1         4 Amp/400 Volt Bridge Rectifier            276-1173
3         Silicon Controlled Rectifier (SCR)         NTE5457
3         120 VAC/ 25 Watt incandescent lamp
1         50-100 microfarad/ 200 volt capacitor
1         1000 microfarad / 35 volt capacitor        272-1032
1         50 ohm resistor/ 5 or 10 Watt              271-133
3         Push Button Switch (normally open)
1         Push Button Switch (normally closed)
3         2K resistor, 1/4 watt                      271-1325
4         1N914 Diode
1         51K resistor, 1 watt
1         2 Amp Fuse                                 270-1064
1         Relay (SPDT) 9 Volt DC, 500 ohm coil       275-005

Ignition Coil Buzz Box

Here's a circuit to create a buzzcoil using a standard automotive ignition coil. A 556 dual timer is used to establish the frequency and duty cycle of the coil current. One of the timers is used as an oscillator to generate the 200 Hz rectangular waveform needed to control the (IRF740 MOSFET) while the second timer switches the oscillator on and off as the breaker points open and close (closed = on). The result is a steady stream of sparks from the ignition coil spaced about 5 milliseconds apart while the breaker points are closed. 

Operation:
Pin 8 and 12 are the threshold and trigger inputs of one timer which are driven by the breaker points and produce an inverted signal at the timer output (pin 9). When the points are closed to ground, pin 9 will be high and visa versa. The signal at pin 9 controls the reset line (pin 4) of the second timer and holds the output at pin 5 low while pin 4 is low and pins 8 and 12 are high (points open). The 15K and 4.7K resistors and 0.33uF capacitor are the timing components that establish the frequecy and duty cycle of the second timer which is about 4 milliseconds for the positive interval and 2 milliseconds for the negative. During the positive time interval, the MOSFET gates are held high which causes the ignition coil current to rise to about 4 amps. This equates to about 80 millijoules of energy in the coil which is released into the spark plug when the timer output (pin 5) moves to ground, turning off the MOSFET. A 12 volt zener diode is placed at the junction of the 10 and 27 ohm resistors to insure the MOSFET gate input never goes above 12 volts or lower than -0.7 volts. A 200 volt/5 watt zener is used at the MOSFET drain to limit the voltage to +200 and lengthen the spark duration. The circuit should operate reliably with a shorted plug, however operating the circuit with no load connected (plug wires fallen off, etc.) may cause a failure due to most of the power being absorbed by the zener. You can also use a transient voltage suppressor (TVS) such as the 1.5KE200A or 1.5KE300A in place of the zener. It's probably a better part, but hard to obtain.

Bistable Flip Flop

Here are two examples of bistable flip flops which can be toggled between states with a single push button. When the button is pressed, the capacitor connected to the base of the conducting transistor will charge to a slightly higher voltage. When the button is released, the same capacitor will discharge back to the previous voltage causing the transistor to turn off. The rising voltage at the collector of the transistor that is turning off causes the opposite transistor to turn on and the circuit remains in a stable state until the next time the button is pressed and released. Note that in the LED circuit, the base current from the conducting transistor flows through the LED that should be off, causing it to illuminate dimly. The base current is around 1 mA and adding a 1K resistor in parallel with the LED will reduce the voltage to about 1 volt which should be low enough to ensure the LED turns completely off.

9 Second Digital Readout Countdown Timer

This circuit provides a visual 9 second delay using a 7 segment digital readout LED. When the switch is closed, the CD4010 up/down counter is preset to 9 and the 555 timer is disabled with the output held high. When the switch is opened, the timer produces an approximate 1 second clock signal, decrementing the counter until the 0 count is reached. When the zero count is reached, the 'carry out' signal at pin 7 of the counter moves low, energizing the 12 volt relay and stopping the clock with a low signal on the reset line (pin 4). The relay will remain energized until the switch is again closed, resetting the counter to 9. The 1 second clock signal from the 555 timer can be adjusted slightly longer or shorter by increasing or decreasing the resistor value at pin 3 of the timer. 

The CD4510 is a CMOS Presettable BCD Up/Down counter which can be preset to any number between 0 and 9 with a high level on the PRESET ENABLE line, (pin 1) or reset to 0 with a high level on the RESET line (pin 9). Inputs for presetting the counter (P1, P2, P3, P4) are on pins (4, 12, 13, 3) respectively. The counter advances up or down on each positive-going clock transition (pin 15) and the counting direction (up or down) is controlled by the logic level on the UP/DOWN input (pin 10, high=up, low=down). The CARRY-IN signal (pin 5) disables the counter with a high logic level. 

The CD4511 is a CMOS BCD to 7 segment latch decoder capable of sourcing up to 25 mA which allows it to drive LEDs and other displays directly. A LATCH-ENABLE line (pin 5, active low) stores data from the BCD input lines. A LAMP-TEST input (pin 3, active low) can be used to illuminate all 7 segments, and a BLANKING input (pin 4, active low) can be used to turn all segments off. The LED display must be a common cathode type so that the segments are illuminated with a positive voltage on their respective connections. Complete data sheets for the CD4510 and CD4511 can be obtained by answer fax from

12 Volt Lamp Dimmer

Here is a 12 volt / 2 amp lamp dimmer that can be used to dim a standard 25 watt automobile brake or backup bulb by controlling the duty cycle of a astable 555 timer oscillator. When the wiper of the potentiometer is at the uppermost position, the capacitor will charge quickly through both 1K resistors and the diode, producing a short positive interval and long negative interval which dims the lamp to near darkness. When the potentiometer wiper is at the lowermost position, the capacitor will charge through both 1K resistors and the 50K potentiometer and discharge through the lower 1K resistor, producing a long positive interval and short negative interval which brightens the lamp to near full intensity. The duty cycle of the 200 Hz square wave can be varied from approximately 5% to 95%. The two circuits below illustrate connecting the lamp to either the positive or negative side of the supply.

1.5 Hour Lamp Fader (Sunset Lamp)

Similar to the one above, the sunset lamp comes on at full brightness and then slowly fades out over 1.5 hours time and stays off until power is recycled.

120 VAC Lamp Dimmer

The full wave phase control circuit below was found in a RCA power circuits book from 1969. The load is placed in series with the AC line and the four diodes provide a full wave rectified voltage to the anode of a SCR. Two small signal transistors are connected in a switch configuration so that when the voltage on the 2.2uF capacitor reaches about 8 volts, the transistors will switch on and discharge the capacitor through the SCR gate causing it to begin conducting. The time delay from the beginning of each half cycle to the point where the SCR switches on is controlled by the 50K resistor which adjusts the time required for the 2uF capacitor to charge to 8 volts. As the resistance is reduced, the time is reduced and the SCR will conduct earlier during each half cycle which applies a greater average voltage across the load. With the resistance set to minimum the SCR will trigger when the voltage rises to about 40 volts or 15 degrees into the cycle. To compensate for component tollerances, the 15K resistor can be adjusted slightly so that the output voltage is near zero when the 50K pot is set to maximum. Increasing the 15K resistor will reduce the setting of the 50K pot for minimum output and visa versa. Be careful not to touch the circuit while it is connected to the AC line.

Voltage To Current Converter

Runing Led IC 4017