One Shot Timer Circuit

[Gary Houston]

Hello Everyone:

I am requesting some help for a timing circuit I need for my model railroad. I need about 10 of these circuits to operate 24VDC, 1.5 amp electromagnets for uncoupling the cars. These electromagnets will be turned on and off through a 24 volt relay. I've looked at commercially available timers, and they are expensive, cheapest I saw was $22. So, I am considering building the timers myself. I have the tools, have built some simple electronic circuits in the past, and I am an electrician by trade.

The source I am dealing with is a 24VDC, 7amp, high quality power supply. What I need to do is have the relay turn on when I push a momentary pushbutton, then turn off after 5 seconds. I believe this is called "one-shot" time delay.

I built the following circuit on an electronics lab board, and it appears to work, but I am certain the voltage on the relay is gradually dropping off instead of turning off all at once. I don't know if this is a bad thing or not, but I am afraid it could lead to diminished contact life. I don't know anything about the transistor except that it is an NPN.

If anyone has suggestions to improve this circuit, or an alternative circuit, I would appreciate the help. Perhaps a 555 Timer circuit could be used?

 

[Roff]

How much current does the relay coil require?

 

[Rolf]

Hello Everyone:

I am requesting some help for a timing circuit I need for my model railroad. I need about 10 of these circuits to operate 24VDC, 1.5 amp electromagnets for uncoupling the cars. These electromagnets will be turned on and off through a 24 volt relay. I've looked at commercially available timers, and they are expensive, cheapest I saw was $22. So, I am considering building the timers myself. I have the tools, have built some simple electronic circuits in the past, and I am an electrician by trade.

The source I am dealing with is a 24VDC, 7amp, high quality power supply. What I need to do is have the relay turn on when I push a momentary pushbutton, then turn off after 5 seconds. I believe this is called "one-shot" time delay.

I built the following circuit on an electronics lab board, and it appears to work, but I am certain the voltage on the relay is gradually dropping off instead of turning off all at once. I don't know if this is a bad thing or not, but I am afraid it could lead to diminished contact life. I don't know anything about the transistor except that it is an NPN.

If anyone has suggestions to improve this circuit, or an alternative circuit, I would appreciate the help. Perhaps a 555 Timer circuit could be used?

This might help:
https://555-timer.clarkson-uk.com/operation/mono.html
You will need a voltage regulator for the 555s or 556s and of cause a suitable transistor from the output to drive your relay. Don't forget to put a clamping diode across the coil.

 

[Gary Houston]

How much current does the relay coil require?

Ron, what I found about the relay coil is that it dissipates .9 watts. The spec sheet didn't have a current rating for the coil. Using the .9 watts and dividing by 24 volts, I get 37.5 milliAmps, I assume this is a close approximation of the coil current.

 

[Gary Houston]

You will need a voltage regulator for the 555s or 556s and of cause a suitable transistor from the output to drive your relay. Don't forget to put a clamping diode across the coil.

That circuit looks good, i.e. fairly simple and "I can build that!"

Now, when you say I need a voltage regulator for the 555, I am guessing that the 24 volts is too much. Can you recommend a voltage regulator or circuit to accomplish that?

Another question would be what particular transistor would work and how the transistor would get connected into the 555 timer circuit: I assume pin 3 would go to the base of the transistor, and the emitter and collector would feed 24 volts to the relay coil, but what other connections would be made? Sorry for my lack of knowledge in this area.

Oh, one other thing, any recommendations on a clamping diode? It would be connected reverse biased across the relay coil?

Thank you for the response!

 

[SPDCHK]

Gary,

Use google and search for **broken link removed**. That's a general purpose adjustable voltage regulator that can handle input voltage of up to 28V DC.
Also use google to search for the data sheet of the 555 timer IC, to see at what voltage level you need to adjust the LM317 to. You should also find many 555 tutorials on the web, and I'm sure that the circuit you require will be there somewhere.

Best of luck.

 

[Russlk]

In your original circuit, the NPN transistor was saved by the slow turn off but using the 555 you will need a diode across the relay coil or the transistor will get fried.

 

[Roff]

I drew this up. It should work. You can use an LM7812 (or any other 7812) in place of the LM78L12. You can also use any type 555 timer.

 

[Russlk]

Try this circuit: (now corrected)

 

[Roff]

Russ, isn't that an astable?

 

[Russlk]

You are right, Ron, the hour was late and I was hasty. I will correct it.

 

[dch222]

Non CMOS 555s will operate up to 15V and will sink or source 200mA so shouldn't need a transistor - will still need the protection diode though.

 

[Gary Houston]

In your original circuit, the NPN transistor was saved by the slow turn off but using the 555 you will need a diode across the relay coil or the transistor will get fried.

Understood. My rudimentary understanding is that the coil will give an inductive spike back through the circuit when it turns off which could damage components. The diode provides a parallel path to protect the circuit. Is that correct?

Also, this lab board I am working on has internal components with protective circuitry inside to keep the components from frying.

 

[Gary Houston]

Ron and Russ:

Thanks for your time and effort, much appreciated. Now I will hunt down the components.

Just to make sure I understand:

Ron: The LM7812 is a three pin voltage regulator that outputs 12 volts?

Russ: Your circuit is using a zener diode as a voltage regulator?

And, is there any need to heat sink any of these components? Probably not?

 

[Gary Houston]

Non CMOS 555s will operate up to 15V and will sink or source 200mA so shouldn't need a transistor - will still need the protection diode though.

Everyone:

Perhaps I should convert the whole thing into a 12 volt operation? I didn't mention it, but the electromagnets I am controlling are built as "dual voltage" units meaning each magnet has two 12volt coils. Right now, they are wired in series and operate on 24 volts and 1.5 amps. I did this because I already had a bunch of 24 volt DC relays.

I could wire the electromagnet coils in parallel and put them on 12 volts at 3 amps. I do have a nice 12 volt DC 5 amp power supply, so this could be done. I would have to get some 12 volt relays. So this would do away with the voltage regulator and the transistor and simplify the circuit?

Once again, I really appreciate the generous and timely respsonses!

 

[Roff]

Everyone:

Perhaps I should convert the whole thing into a 12 volt operation? I didn't mention it, but the electromagnets I am controlling are built as "dual voltage" units meaning each magnet has two 12volt coils. Right now, they are wired in series and operate on 24 volts and 1.5 amps. I did this because I already had a bunch of 24 volt DC relays.

I could wire the electromagnet coils in parallel and put them on 12 volts at 3 amps. I do have a nice 12 volt DC 5 amp power supply, so this could be done. I would have to get some 12 volt relays. So this would do away with the voltage regulator and the transistor and simplify the circuit?

Once again, I really appreciate the generous and timely respsonses!

As you say, 12 volt operation will greatly simplify the circuit. You should put a big cap (at least 100uF - maybe more) on the circuit board between +12V and GND to soak up the current transients. A 100nF in parallel will help to handle the higher frequency components that the electrolytic can't handle.

 

[Gary Houston]

Here's my guess at what the 12 volt circuit would look like. I removed the voltage regulator and the transistor, and added the capacitors.

I'm not sure whether the relay coil is connected properly. Does it simply get fed from the 555 output as I have drawn?

 

[Roff]

Here's my guess at what the 12 volt circuit would look like. I removed the voltage regulator and the transistor, and added the capacitors.

I'm not sure whether the relay coil is connected properly. Does it simply get fed from the 555 output as I have drawn?

The 555 puts out a positive pulse. See below.

 

[Gary Houston]

I was wondering whether the 555 switched the positive or negative. Thank you for the diagram and for your time. I'll put this together sometime soon and let y'all know how it turns out.

 

[Hero999]

A 555 can act as either a current source or sink.

The 555 is more commonly used as a current sink for low duty cycle applications and as a source for high duty cycle applications because the minimum duty cycle without a diode is 50%