# Pulse (One-Shot) Timer

Discussion in 'General Electronics Chat' started by Kal_B, Mar 12, 2017.

1. ### Kal_BMember

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Hi guys,

I'm working on this small project for a pulse timer. I have a small machine with NO switch that I want to connect to the timer . This way the switch will produce a rising edge and the contacts will have to open and then closed again for another rising edge. So if the switch contacts are held closed forever the timer would produce only one pulse to energize a relay and hold it for a specified period (Determined by R2 and C1) and then release.

Part of the circuit (the part with the 555 timer) is off the net and I'm not quite sure how it works. Specifically C3,R5 and D1 were added so that the switch being closed continuously would have no effect on the 555. It was described this way in the circuit instruction:
"a capacitor can be used to isolate the switch so that only the initial switch closure is seen by the timer input and the switch can remain closed for an unlimited period without effecting the output".

Thanks
Kal

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2. ### AnalogKidWell-Known Member

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1. There has to be a connection between the X3 input diode cathode and the 555 GND.

2. Since the 555 and the load are powered by the same source, what is the reason for X3, the optocoupler?

C3 and R5 form a differentiator circuit or network. This "extracts" the leading edge of the pulse signal coming from the switch. No matter how long you hold the switch closed, R5 charges C3 producing an exponential ramp voltage across C3. This ramp is characterized a single number to indicate how fast it increases. That number is the time constant, t:

t = R x C (R is resistance in ohms, C is capacitance in farads)

With the values shown, the time constant is 10 ms. This means that the capacitor charges up to the 63% of Vcc in 10 ms, 86% in 20 ms, 95% in 30 ms:

https://en.wikipedia.org/wiki/Time_constant#/media/File:Series_RC_capacitor_voltage.svg

So no matter how long you hold down the button, the 555 sees a pulse of approx 11 ms (1.1RC).

ak

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3. ### alec_tWell-Known MemberMost Helpful Member

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24V exceeds the maximum supply voltage for the 555. You could add a 1k resistor from pins 4 and 8 to ground to reduce the voltage.
The relay should have a suppression diode across its coil.

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5. ### AnalogKidWell-Known Member

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I missed the supply voltage. Yes, add a 12 V Zener diode across the 555 power pins and feed it with a 1 K resistor. Also, I strongly recommend changing to a CMOS 555, such as an LMC555. If one end of the relay does not have to be connect to GND, you can re-arrange some things and delete a bunch of parts:

1. Change the MOSFET to n-channel
2. Delete R6, X3, R10, Q3, R7, R8, and R9.
3. Connect 555 pin 3 to the MOSFET gate.
4. Connect the relay between Vcc and the drain.
5. Connect the source to GND.

ak

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6. ### Kal_BMember

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Thanks AnalogKid and alec_t.

The optocoupler was poorly thought out by me and I was looking for is what alec_t recommended which a freewheeling diode( I hope I got it right).
I'll update the schematic and post the new one.

Thanks a lot guys

Kal

7. ### Kal_BMember

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Here is the new schematic and of course more questions

-I have LMC555 and also LM55CN which one is better ad why?
-I changed the values of R2 and C1 to and I'm likely to to use a 100K pot for R2. I used this calculator to determine RC time constant and one of the fields required "Output Voltage" which I'm not entirely sure what it is. Is it the voltage that will be going into the timer chip?
-The one 1K resistor going from 4 and 8 to ground alec_t recommended is in addition to the 1K resistor I have going from VCC to both 4 and 8 pins (as in version 3)?

Cheers
Kal

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8. ### alec_tWell-Known MemberMost Helpful Member

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Yes. As you have it in Version 3. But AK's suggestion is better. Replace that R3 with a 12V zener diode.

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9. ### AnalogKidWell-Known Member

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R1, R5, and D1 connect to the bottom end of R4. This keeps the max voltage on all 555 pins within spec.
Add a 0.1 uF ceramic and 10 uF electrolytic in parallel with D2. These are called decoupling capacitors, and do several things to keep the chip happy.
The schematic is missing some connection dots. I'm not familiar with Eagle, so they might not be a problem in terms of porting the schematic to a layout editor.

ak

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10. ### Kal_BMember

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Yes the software is quite challenging and nightmarish at times. I've been struggling with it all day and pretty much gave up on it and most likely will go back to LTSpice.
I had two caps at the Voltage source as decouplers but forgot to put in the values, but I added the two caps you recommended at D2

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11. ### AnalogKidWell-Known Member

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Looks good to me.

ak

12. ### eTechActive Member

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Hi

Move the +24 connection of the timing resistor R2 to the VCC connection of the 555 so it is also operating "in spec" with the NE555 max supply voltage requirement.
Connect a 0.1uF cap across pin 4(GND) and pin 8(VCC) of the 555 to prevent spurious noise from affecting the 555.

eT

Last edited: Mar 14, 2017
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13. ### AnalogKidWell-Known Member

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Missed that. Yes, the top end of R2 needs to be connected to the zener diode.

ak

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14. ### Kal_BMember

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An interesting thing has been happening. The timer energizes a 24VDC d that has a 220VAC running through one of it's normally open contacts. When I close the switch to activate the timer, the relay oscillates back and forth, On and OFF. If I remove one of the 220VAC wires the relay works as it's expected, being turned on by the timer for a bout a second and then turning off. Is the phenomenon is a result of induction? If yes, would and SSR resolve that issue?

Edit: I tried SSR and had the same results.
Thanks
Kal

Last edited: Mar 15, 2017
15. ### AnalogKidWell-Known Member

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Could be miswiring. A detailed wiring diagram or schematic would be nice, plus photos.

ak

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16. ### Kal_BMember

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Thanks AK. I will as soon as I get a chance

17. ### Kal_BMember

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Here are two circuits that I tried and produced the same results. When the switch is close, no matter how long or short, both relays oscillate continously going on and off until I unplug at R or the DC power. Everything works fine with R removed from circuit (unplugged form its base)
The DC relays are DPDT Allen Bradley. The contcator I think is Siemens. There's nothing connected to R contacts.

Considering the simplicity of the circuit I'm inclined to believe that I incorrectly wired the timer on the PCB and have reviewed it twice and will look at it today again to find where I went wrong.
Thanks
Kal

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18. ### shortbus=Well-Known Member

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If the original 'triggering' switch is mechanical, are you debouncing it? Most mechanical switches need it.

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You may want to place a RC Snubber across the relay contacts. The link is just an example of those commercially available, you can roll your own using a resistor and capacitor in series across the relay contacts. A Google of RC Snubber should get you all you need for your application. I would give a snubber a try.

Designing RC Snubber Networks

Ron

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20. ### AnalogKidWell-Known Member

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It's a 5 second monostable. I don't think a little switch bounce will matter. Plus, the trigger input sets a flipflop. Unless the bouncing extends for more than the timer period, everything after the initial edge is ignored.

ak

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21. ### Kal_BMember

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Thanks guys, this has been and interesting one. I have been experimenting with it to figure out the problem and here's an updated circuit.
First few notes:
1- R2 is actually a 100K pot.
2- If I wire a 24VDC LED in place of R2 everything works just fantastic which leads me to believe that the problem is that of induction.

The new circuit is exactly like the old ones with the exception that I am using a separate power supply for R2 just to eliminate possible issues.
Observations:
1-I adjusted the pot so that the time is for a bout 2 seconds so I can see what is happening sort of slow motion.
2- With SW2 OFF, closing SW1 energizes the relay immediately and after about two seconds the relay de-energizes. If the SW1 is held closed for the entire period or longer or shorter it doesn't matter. In other words the circuit works exactly as it suppose to.
3- With SW2 ON, closing SW1 starts the same sequence as above but when the timer times off and SW2 de-energizes the falling edge of SW2 energizes the timer again somehow (my guess through induction or the removal of induction) and the sequence restarts. Some times this keeps going on and on and sometimes it stops after a few times. While it's oscillating if I turn off SW2 the sequence stops.

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