Help with low-pass filter and TTL logic interface

Question

I am working with a motor control board which controls a DC motor (speed and direction). This board uses two SPDT relays for direction, and some sort of PWM for speed.

Here's an abbreviated schematic, including my own circuit.

I need to feed the motor direction to a MCU using TTL levels.

I tapped the circuit at point "A", which outputs 0V when relay is not energized and 0.8V when energized. This corresponds with the motor directions.

I amplified this signal to TTL levels using the circuit shown.

This works perfectly, except when the motor is running slowly (heavy PWM applied to motor outputs). In this state there are repeated spikes at point A and B (see this example) which cause the MCU to think the pin has changed state.

You can see the low-pass filter intended to attenuate anything above 2 Hz. But it doesn't filter these spikes for some reason.

Does anyone have any suggestions for cleaning up this signal?

The controller and my circuit share a common ground.

Thank you for any advice!

Answer 1

So I solved the problem. I built an opto-isolator using an IRED and phototransistor and some black tubing. This effectively eliminates the EMI and gives me a clean square wave. Note that some applications may require a resistor on the base of Q1, but in this application the resistor is already present on the control board as shown in the previous schematic.

For the phototransistor I used some left-over GE L14GX536 parts in my bin.

Here's a schematic of the final product, in case it's helpful to someone:

Answer 2

Is the problem related to sharing the base resistor between two transistors?

^{simulate this circuit – Schematic created using CircuitLab}

Case 1 shows your present configuration. Is it possible that one transistor hogs the base current under certain circumstances causing unreliable switching of Q2?

Case 2 shows independent base drive for each transistor. This should be more reliable.

Case 3 is just checking the obvious: Why not take the control board directly to the MCU? I presume the answer is that it's not TTL output levels.

Answer 3

Tapping the signal at point "A" was probably motivated by the idea of protecting the ASIC (U1) from possible damage. But it says, "TTL-Level Outputs". So it's very likely that output could connect directly to the MCU.

If you still want to be cautious, move point "A" to the other side of the resistor, and add another series resistor to drive your transistor, Q1. This provides a better drive signal to Q1. Doing it the way you show essentially tries to drive two transistors in parallel, each of which is going to try to limit the "high" state voltage to (as you observed) 0.8 volts. They would be sharing the input current from the single resistor.

One more thing. Because of the transistor's gain, or change in impedance going from input to output, it's much harder to filter the signal at the output. Filtering the input (at the base of Q1) would require a smaller capacitor. The impedance is higher because of the decoupling from the input resistor. Probably with the changed drive setup, you won't need any filtering...assuming there isn't any PWM present on that direction signal from U1.