Hi Indecided,
I see a few things wrong that can cause your problem:
1) If you look closely at your "reference" circuit, it has a 7.5V zener diode at pin 9 of the LM29xx. That zener diode is included inside an LM2917, but your LM2905 doesn't have one. Add one to your circuit and it will probably work much better.
- note taken
2) The circuit that you are using has an extra opamp-comparator and relay driver. Why is it using a lousy old 741 opamp when the LM2905 has a very good comparator and powerful output transistor built right in?
The 741 requires its inputs to be at least 3V away from its positive supply and ground (its negative supply), which is called its common-mode input voltage range. Your circuit has its inputs too close to ground. The opamp/comparator inside your LM2905 works very well with its inputs all the way down to ground.
The 741 is guaranteed to drive a 2Kohm load to only 10V, when using a dual 15V supply. That is a loss of 5V with a load current of only 5mA. The output transistor inside the LM2905 has a low loss and can drive a load current of 50mA.
- i was unaware of the 741's requirements... the original voltage designed for should have been... about 2.84v at 4000rpm. however... how do i harness the op-amp/comparator when utilizing a voltage output?
3) The datasheet shows a 100Kohm resistor at pin 3 for an 8 cylinder engine and a 133Kohm one for 6 cylinders. Therefore use 200Kohms for 4 cylinders.
- That's my probelm here. I'm currently using a 250k pot.. and at full resistance, i get about 0.6 volts at 3000RPM. should be about 2v+..
Here i have experimented with a 1meg pot. I get the voltage i want, however, the latency experienced is unacceptable (i have to wait for the voltage to rise and fall when i throttle rapidly)
4) In addition to getting rid of the old 741 opamp, the LM2905's datasheet shows how to connect its opamp/comparator as a speed switch, which is what you want. Use a 10Kohm pot instead of its 5Kohm resistors to adjust its trip point. It might be accurate enough without a zener diode. It is using the 8-pin IC, the pin numbers are different if yours has 14 pins. Their circuit is here:
- The problem however.. is that is a speed switch. Useful for a door actuator lock at speed, or a speed limiter. The problem here however, is that i need to connect it as a REV comparator. not a speed.. so when I change gear i get notification.. not at a certain speed!