# How to turn on an LED when voltage drops to low?

I have an old vehicle from the 70's and it has a gauge for oil pressure. I want to add a 12v LED to the dash that will light up when the oil pressure is too low.

The installed sensor on the engine is restive to ground. The resistance changes as the oil pressure changes (more pressure = less resistance), thus changing the reading on the dash gauge. I want my warning light to be connected to this same sensor.

Note, although vehicle is 12 V, it has a factory "constant voltage unit" that provides the gauges with 5 V. I will be replacing this with a simple 5 V regulator (eg LM7805). So gauge circuit will be... +12v----LM7805----Gauge----Sensor----Ground. The circuit for the LED indicator would connect between the gauge and sensor.

In the past, I've been able to accomplish this with an Arduino and some code, but don't want to use an MCU in this application. However, I can't seem to figure out how to do this with a simple discreet component design.

Thanks!

• Can you give a couple of voltage readings across the sensor for engine off and normal running at least? Hit the edit link. – Transistor Apr 28 at 11:01
• Does "restive" mean negative or positive? – Russell McMahon Apr 28 at 11:04
• Would be cooler if you had a new vehicle from the 70's. – pipe Apr 28 at 13:31
• Similar question answered here. – Pete Becker May 5 at 12:16

An Arduino, at a cost of under \$5, is an excellent solution.

This circuit should work, but values will depend somewhat on the oil pressure sensor resistance.

Reducing sensor voltage below a certain level will turn on Q1 and so LED1.
R2 can be adjusted to vary the turn on level.

The circuit can be improved if of interest.

simulate this circuit – Schematic created using CircuitLab

• Thanks! I tracked down a wiring diagram and (like @Reroute mentioned) noticed it has a voltage regulator for the gauges. I'm going to be replacing it with a simple 5v regulator (eg LM 7805). So gauge circuit will be like this... +12v-----LM7805-----Gauge----Sensor----Ground How would your circuit be changed, if at all, given the change to 5v? – Hoss May 5 at 11:46

For a vehicle from the 70's the resistance is probably in the ballpark of 10 ohms high reading, 73 ohms low reading, and depending on the type of gauge may by using a bimetallic regulator to give the gauge an average of 5V, which means this signal may be pulsing between off and a signal at 1-3Hz,

If the gauge comes off the low reading with small pulses, and takes about 5-6 seconds to reach a mid reading its this type of setup, and you will need to design around the regulator, or replace it with a modern 5V regulator to make it consistant (some gauges are 7V, but most are 5V in that era)

If its the regulator type, then lets say 1 op amp as a peak detector to catch it while its switch on, and a second as a comparitor to turn on the LED when the voltage is too low.