Oil Temperature Gauge for 125 cc Scooter

Oil Temperature Gauge Circuit Diagram :

• no moving parts (so not meter movement), as scooters vibrate a lot!;
• as cheap as possible (around £12);
• robust measuring transducer (avoid NTC thermistors and other ‘exotic’ sensors);
• temperature range 50–140 °C. (122 – 291 °F);
• audible and visual warning in case of dangerous temperature;
• compact;
• waterproof.

Let’s start by the sensor. This is a type-K thermocouple, as regularly used by multimeter manufacturers. Readily available and fairly cheap, these are robust and have excellent linearity over the measurement range we’re interested in here. The range extends from 2 mV to 5.7 mV for ten measurement points. The positive output from the thermocouple is applied to the non-inverting input of IC3.A,  wired as a non-inverting amplifier. Its gain  of 221 is determined by R1 and R2. IC3 is an LM358, chosen for its favourable characteristics when run from a single-rail supply. IC3.B is wired as a follower, just to avoid leaving it powered with its pins floating.

IC3.B output is connected to pin 5 of IC1, an LM3914. This very common IC is an LED display driver. We can choose ‘point’ or ‘bar’ mode operation, according to how pin 9 is connected. Connected as here to the + rail, the display will be in ‘bar’ mode. Pin 8, connected to ground, sets the full scale to 1.25 V. R3 sets the average LED current. Pin 4, via the potential divider R7/R8+R9, sets the offset  to 0.35 V. Using R8 and R9 in series like this avoids the need for precision resistors.

As per the LM3914 application sheet , R4-R5-R6 and C5 will make the whole display flash as soon as D10 lights (130 °C = 226 °F). Simultaneously, via R10 and T1, the (active) sounder will warn the user of overheating. Capacitor C6 avoids undesirable variations in the reference voltage in ‘flashing’ mode. IC2 is a conventional 7808 regulator and C1– C4 filter the supply rails. Do not leave these out! D1 protects the circuit against reverse polarity.