Feedback on 555 timer Schmidt Trigger design

So now that I know it is for model railroad sensing... have you considered capacitive sensing? The trains are metal, at least as far as the trucks, yes?

I haven't looked at capacitive sensing. I did try hal-effect. The locomotive motor is not sensed by the transistor I have. Plan B place magnet on bottom of car. This works but if the train coast it leaves the sensor after triggering. It also requires modification to the cars by placing a magnet.

I have also tried current sense. The Design Challenge gave me a sensor to test that worked. In a DC train powered system the track voltage can be reversed. The sensor doesn't like it.

I have successfully used the IR sensor to create a scale speed testing device. It worked pretty good until I turned on the halogen light and then it went for a crap.

colporteur said:

I have also tried current sense. The Design Challenge gave me a sensor to test that worked. In a DC train powered system the track voltage can be reversed. The sensor doesn't like it.

can a rectifier fix that?

I've used this circuit for remote capacitive sensing. It returns a pulse width proportional to capacitance between the sensor plate and ground. It is described here as a level sensor, but it also works as a body sensor, touch sensor, basically anything conductive that gets close will cause the pulse width to get longer.

https://hackaday.io/project/183506-remote-continuous-liquid-level-sensor-555-timer

To use: connect to one pin of an Arduino. 

Now just time how long it stays high between low pulses.

The sensor should be two small copper plates or foil next to each other under the train, one connected to the sensor input, the other to circuit ground. Do NOT ground the sensor at the track, only back at the Arduino.

You'll just have to test it and see how sensitive it is in your case. Larger copper plates will be more sensitive. Make both plates the same size.

Is this the sensor placement you are suggesting?

Is this the sensor placement you are suggesting?

That should work. Remember, though, don't ground it to the track. The 555 circuit can sit right under the copper pieces, so there is very little wire to add parasitic capacitance.

You may have the ability to measure capacitance with your DMM. Usually the lowest range is in nF, but 0.001nF = 1pF. You can get a rough idea that way of how much change there is.

Or build my device that uses two 555 timers to measure capacitance. It uses the display on your DMM, where on the lowest range, 1pF is 1mV. On the lowest scale, most DMMs can measure down to 0.1mV, so you can have 0.1pF fine grained measurements.

https://hackaday.io/project/183405-dual-tlc555-capacitance-meter-01pf-resolution

How accurate depends on the capacitor you use to calibrated it. Let's face it, 5% is fine for 99.999% of what we do, so a capacitor between 100 and 200pF is good to go, even if it is 5% or better tolerance. Lots of mica capacitors in that range are 2 or 3% tolerance.