LCD-LED DisplaySensors - Tranducers Circuits

Oil Temperature Gauge Schematic Circuit Diagram

Building a Temperature Gauge for Far-Eastern Scooters with GY6 Engines

Many Far-Eastern scooters come equipped with GY6 engines, which, despite their age, are robust and fuel-efficient. However, for those seeking enhanced performance through modifications like ‘Racing’ kits and improved advance handling, the issue of engine temperature arises. To tackle this challenge, adding a heatsink (often mistaken as a ‘radiator’) to the oil circuit becomes necessary. Even with this modification, users require a reliable method to monitor oil temperature. We established specific criteria for the temperature gauge we aimed to develop:

Oil Temperature Gauge Schematic Circuit Diagram

  • no moving parts (so no 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.

Creating a Temperature Sensor Circuit

The sensor used in this setup is a type-K thermocouple, a standard choice among multimeter manufacturers. These thermocouples are easily accessible and cost-effective. They offer robustness and excellent linearity within our desired measurement range, ranging from 2 mV to 5.7 mV across ten points. The positive output of the thermocouple is connected to the non-inverting input of IC3.A, configured as a non-inverting amplifier. The gain, set by resistors R1 and R2, is 221. IC3, an LM358, was chosen due to its favorable characteristics when operating on a single-rail supply. IC3.B is configured as a follower to prevent its pins from floating without power.

Implementing LED Display and Overheating Warning System

IC3.B’s output is linked to pin 5 of IC1, an LM3914 LED display driver commonly used in electronic projects. The operation mode, whether ‘point’ or ‘bar,’ is determined by the connection at pin 9. In this configuration, connected to the + rail, the display operates in ‘bar’ mode. Pin 8, grounded, sets the full scale to 1.25 V. R3 establishes the average LED current, and pin 4, through the potential divider R7/R8+R9, sets the offset to 0.35 V. To make the entire display flash when D10 lights up at 130 °C (226 °F), resistors R4, R5, R6, and capacitor C5, as per LM3914 application guidelines, are incorporated. Simultaneously, through R10 and T1, the active sounder alerts the user about overheating. Capacitor C6 ensures the reference voltage remains stable during ‘flashing’ mode.

Ensuring Stability and Protection

IC2 serves as a standard 7808 regulator, and capacitors C1 to C4 filter the supply rails, a crucial step to maintain stability. It is imperative not to omit these components. D1 safeguards the circuit against reverse polarity, preventing potential damage. To facilitate assembly, the author has designed two PCBs that can be arranged as a ‘sandwich,’ with the CAD file available for download [1]. The download package also includes a document featuring project photos. Notably, hot-melt glue is utilized as a secure bonding agent, proving superior to epoxy due to its reversibility and effectiveness against vibrations.

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