Momentary pulse from dark detector circuit

Dick-

Dallee offers a timed station stop module to detect trolleys and other locomotives, and I think it has outputs to trigger accessory functions too. It might work for your application.

-John

Dick,

Look up the 555 IC used as a Monostable Multivibrator.

Dave

As the trolley stop scenario is a common application, I'd think one of the off-the-shelf modules like the Dallee mentioned above would be the expedient solution. 

But if experimenting and learning more about electronics is part of the hobby for you, there are many ways to do this.  You need to provide more information about what the delay circuit requires by way of a pulse (voltage, duration, etc.).  In the simplest implementation a single coupling capacitor can generate a pulse (spike) from the output of a comparator to trigger a downstream circuit.  So again it depends.

For example you mention a quad comparator.  You will have some spares so you can also wire up a comparator to be a timer which might eliminate the need for the timer module.  You can wire up a comparator to generate a pulse (so-called "one-shot" timer).  And so on. 

The 555 IC is another good building block with the distinction of relatively high output current for an IC which might eliminate the need for a relay or driver transistor depending on your application.

Stan2004 is right. I want to experiment and learn about electronics as a hobby. It would be quicker, cheaper and more reliable to buy the Dallee, but this is more fun for me.

I have seen a number of timing circuits, but they all operate off a momentary push button switch. What I want to do is operate on of those circuits when a light sensor is covered. I am using a bump and run tolley and want to stop it at one end (after it bumps to switch direction. The trolley would end-up stopping over the sensor for a period of time (maybe 30-60 seconds).That would keep the light detector circuit sending a steady pulse, not a momentary one.

I would run the trolley track power through a relay which is controlled by a timing circuit, shutting down power while the trolley is at the station. I have used a series of diodes to cut the trolley power to 12 - 14 volts so it doesn't go to fast. (Thanks, Dale M).

I think if i could figure out how to emulate a momentary switch on the transition of the light detector circuit from low to high (or vis versa), I would be in  good shape.

Thanks for the great resource. Looks like I have some studying to do.

stan2004 -- Thanks for the link to the Doctronics site. I have been working with their tutorials and have progressed to the point where I have created a circuit using a 555 that will run an led for 24 seconds, then turn it off. The next step is to add a transistor to open a NO relay to cut off power to the trolley track.

I am having a ball and learning alot. The link was a great resource.

If the relay coil current requirement is not too high, you can directly drive it from the 555 output (pin 3).  This link shows the hookup with the two cases of driving the relay when pin 3 is "high" or when pin 3 is "low".   If you don't know the coil current requirement, you can measure the coil's resistance and use Current=Voltage/Resistance.  So if you measure, say, 400 ohms, and you're using 12V DC, then Current = 12V / 400 ohms = 0.03 Amps = 30 mA which would be well within the 555's direct output capability.  I chose 400 ohms as an example since that is the resistance of the lowest cost 12V SPDT, 10 Amp contact relay I found at DigiKey ($1.15).

Of course since a driver transistor and resistor runs less than 25 cents, it's good to experiment since I guarantee that at some point you will use a 555 IC to drive something that requires more current!

Pretty impressive how quickly you're coming up the learning curve!