Phidgets interface with Dead Strip

[newimaging]

I am trying to get a Phidgets interface to work with a dead strip setup.
When touching the dead strip with a 9V battery the lap counter on PCLapTimer works perfect.
But car`s don`t seem to generate the required voltage to trigger (4V-30V)
Has anyone found a solution to this or build a amplifier.
Also, can anyone tell me about how much voltage a car motor generates. Mainly looking at Slotit 1/32 and Scaleauto or BRM 1/24th with stock motors.
I tried measuring with a volt meter, but it does`t have enough time to get the reading, I only get about 0.065V, and I don`t have an oscilloscope.

Thanks,

Christian

[Gone Racin]

The voltage a motor generates depends on how fast its going.
A few volts at low - medium speed is typical.
The voltage isn't constant, the shape of the waveform depends on brush overlap and will have noise on it.

It is possible the length of contact as well as the voltage could be an issue. A simple transistor / capacitor circuit is sometimes used to spread the pulse so that this isn't a problem.

[2FER SLOTS]

I was of the understanding that a dead strip worked as a motor "shorted" the two rails.
This would make the small voltage the motor produced a troublesome byproduct.

[newimaging]

I am trying to use the motor generated voltage as a voltage input for my Phiget board. But on my multimeter I am only getting about 0.065V, but it might be to short of a pulse for the meter to register it. But I don`t think its 4V, otherwise the board would be recognizing it. It does recognize when I tap a 9V battery onto the deadstrip very quickly.
I tried building an op amplifier today, but there was to much noise, so I have to play with that some more.

[Gone Racin]

Originally Posted by newimaging

I am trying to use the motor generated voltage as a voltage input for my Phiget board. But on my multimeter I am only getting about 0.065V, but it might be to short of a pulse for the meter to register it. But I don`t think its 4V, otherwise the board would be recognizing it. It does recognize when I tap a 9V battery onto the deadstrip very quickly.
I tried building an op amplifier today, but there was to much noise, so I have to play with that some more.

Dead strip counters are normally designed so the car will still count however slowly it crosses the counter strips. At the risk of stating the obvious, cars that go over the counters a bit slow still deserve to have their lap counted. Motors running slowly produce very little back emf, so simply using the motor generated voltage as a voltage input to the counters will mean the car won't count below a certain minimum speed.

The back emf from the motor will be well over 0.065V unless it's running very slowly.
The voltage will depend on how fast the motor is runing and the motor characteristics.
How fast the motor is running in a car going over the counter strips at full racing speed depends on where the counter strips are sited. It's normal practice to mount them on slow-medium speed part of a straight, so the back emf will be well below the value at maximum speed.
I've done a few tests, and at the sort of speeds the cars go over dead strips the maximum of the waveform is in the range 2 or 5 volts.

Originally Posted by 2FER SLOTS

I was of the understanding that a dead strip worked as a motor "shorted" the two rails.
This would make the small voltage the motor produced a troublesome byproduct.

Yes it is normal to design dead strips to work when the two rails are shorted. This means the car will still count even if the car is just put on the counter strip unpowered (that's a handy way of testing them).
However, counters are useless unless they can cope with cars passing over them at normal racing speed. When moving at racing speed, as well as the back emf quite a few cars bounce slightly going over the counter strip causing a somewhat intermittent signal. This is what is happening so unless the counter design can cope with that reality they won't work reliably.

[newimaging]

Well, I ended up building 4 Op Amplifiers to boost the signal from the cars. The nice thing is that now I can push them at 1mph or less, and it still counts the laps. Very reliable. I ended up building it around the LM324 chip, which has 4 amp circuits. I added a diode, so that I didn`t have to create the gap in front of and behind the dead strip. So my total dead strip zone is 2.5"
I have about 2' of cable between the amp and the Phiget board, and am not experiencing noise issues, after adding a small capacitor and resistor on the input. I`ll try to make a drawing of the circuit to share. When I tested it, I had a 60' cable running into the house to my computer, and even with the long cable (antenna) before the amp the readings worked great.
I was told to expect about 80% of energy when using the motor as a generator, so the 3V-5V sounds right.

Christian