I am trying to make an op-amp based circuit to read the tach signal from the ECU in my car aqnd supply the signal to a z8-based tachometer that I am building. (hooking the input pin direct to the tach signal wire made the tach in the dah stop working, so I'm using the op-amp as a buffer)
I have a working circuit as below;
**broken link removed**
When I use a 741 single op-amp it works great, giving the following outputs;
VS 3.3v = VO 1.08v
VS 0v = VO 2.57v
R1 and R2 are both 100k
I have a working prototype of this circuit hooked up to my car - works great, dash tach readout is good and my z8-based tach read correctly etc.
The problem comes when I try to is an LM747CN (dual opamp, because I'm also going to read the vehicle speed), the same opamp inverting buffer circuit gives me these outputs;
VS 3.3v = VO 1.93v
VS 0v = VO 2.64v
The problem is that 1.93v is too high to be seen as a logical low by my Z8-based circuit (input pin confiogured for falling-edge interrupt).
I have tried 2 different LM747's and get the same result. How do I get the correct output range?
Any ideas?
I have a working circuit as below;
**broken link removed**
When I use a 741 single op-amp it works great, giving the following outputs;
VS 3.3v = VO 1.08v
VS 0v = VO 2.57v
R1 and R2 are both 100k
I have a working prototype of this circuit hooked up to my car - works great, dash tach readout is good and my z8-based tach read correctly etc.
The problem comes when I try to is an LM747CN (dual opamp, because I'm also going to read the vehicle speed), the same opamp inverting buffer circuit gives me these outputs;
VS 3.3v = VO 1.93v
VS 0v = VO 2.64v
The problem is that 1.93v is too high to be seen as a logical low by my Z8-based circuit (input pin confiogured for falling-edge interrupt).
I have tried 2 different LM747's and get the same result. How do I get the correct output range?
Any ideas?