Timer circuit that can ignore a persistent trigger current.

I hope it works ok, its just my best guess at what would work.

Let us know what happens. When are you building it?
Hi Fleck, thanks for that and I have gone ahead and built your design and rechecked all the connections.
Before you suggested the extra 1k resistor, I got the 555's LED flashing happily away but there didn't seem to be a whole heap happening with the relay (it didn't switch on) until, that is, I added the additional 1k resistor.
Having added the 1k resistor, although the relay didn't switch on it did, at least, make a humming sound and even clicked very briefly. This could only be due to some current flowing so, maybe the coil is a bit too thirsty for this circuit (it is only a small one but still too big).
So, would that be the reason you'd recommend a transistor instead?
Do transistors demand less current to flow in order to switch on or off?
I don't want to step in on the conversation you guys are having so I am sorry if unwanted. Tim you can turn on a transistor by simply licking your fingers and holding the Base. That is how they amplify so greatly.
Hi George, please join in - ideas welcome and I had no idea as to the low current transistors can respond to!
there is a remote possibility that this circuit might just be working! I've assembled it as per the diagram below and I'm testing it right now, waiting for the 4020 to turn off the diode on the right. Unfortunately it's remaining stubbornly on for now, but a moment ago it did turn off but I'm beginning to think that a loose wire on the breadboard was to blame!
Test Circuit on pin 6.png

I've moved the connections from Q14 (pin 3) to Q7 (pin 6) in the hope that this will reduce the time it takes for the BC 547 transistor to switch off (I didn't want to wait an hour or so for Q14 to do it!).
Am I right with that? - feel like I'm feeling my way in the dark here but still, I'll leave it running for a short while to see if the 547 turns off.
It's late, so I'll catch up with you guys later.
Oh! BTW, to reset the circuit I disconnect the power supply, of course, but roughly how long is needed for it to discharge itself before I can re-apply the power?
You haven't got any base current resistor on the bc547 so when the top transistor switched on it might have blown the bc547. Remember the base current they require is tiny, 1k would be ok again. Also put a 10k resistor from the base to ground to make sure the 547 switches off fully when the 557 does.
Better idea: Why don't you just put the resistor and LED between the bc 557 collector and ground and do away with the 547?
Thanks for the amendments and, as you can see from the diagram below, I've put the relay back in for the time being (I like to hear something go click, I think!).
As for the circuit, Fleck, you were right the 557 had blown and, since applying your suggestions, that part of the circuit works but unfortunately the 4020 isn't doing it's job I think.

Test Circuit on pin 6.png

When power is applied the 555 timer circuit-LED flashes nicely and the relay closes but remains so regardless of the activity (if any) in the 4020.
By closing the switch that simulates pin 6 going high the relay circuit switches off which shows that that part of the circuit works.
  1. Have I wired the 4020 incorrectly?
  2. Is pin 6 (Q7) a pin that will, at some point, go high? (I'm assuming it should trip sooner than pin 14, so I don't have to wait too long)
  3. Maybe the 4020 has blown?
  4. Can the 4020 be tested (easily) to show it's still working, or is it a matter of simply replacing it?
Did I miss something here? I ask because you don't have pin 7 on the 555 hooked up.
Ah!, okay, I notice your earlier wiring diagram for the 555 shows pin 7 connected to the 12V rail via a 4.55M resistor. I would've followed that had it not been for Fleck's diagram that shows it to be unconnected (as is pin 5 on his diagram too).
I have never wired a 555 with pin7 unconnected before. It is very obvious that Fleck knows his stuff that's for sure. So the 555 works with the 7 pin unused?
Thank you Fleck and Tim! I just hooked one up (on the sim) without the 7 pin connected, cool. I always assumed the 7 pin needed to be connected.
Right, I'm home. I had to go to work today & the 3g signal was very poor.

Tim, how fast is the clock pulse? At 1.5 seconds per flash (ie 3/4hz) you'll be waiting about 13min.

You could put a 150 ohm resistor and an led on output 1 to check the counter is working

Also is there a specific reason for having the bc547 transistor? Can't the 557 next to it switch the relay coil directly?
Hi Fleck, thanks for letting me know how to use output 1 to show that the 4020 is counting.
With reference to the BC547, it's there because, without it, the relay didn't seem to have enough power to operate so, I appreciate that this is overkill but please bear with me on this one until I get to try out your suggestion about using a transisor insttead.
Right now I can see that my 4020 is counting nicely, and at a frequency of about 3hz.
I've had Q7 (pin 6) connected instead of Q14 to avoid a long wait but now that you've mentioned it, 13 mins isn't too long and, I guess with a pulse of ~3hz, it'll only be a few minutes.
So, I've now changed it back to using Q14 as the connection to the bases of the PNP transistors. I'm re-running it and, so far (10 mins in and counting) it's not switched itself off.
Thus far with repeated runs, albeit on Q7, I've not yet observed the device switch itself off!
Any ideas?
I would put a 150 ohm resistor & led on whatever output you're using so you can see when it goes high. Then measure the base voltage on the 557 transistors. I'm thinking it might not be high enough to switch them off with the 1k resistors on the bases. You should be able to do away with them as there are resistors in the collectors that will limit the current.
It should be high enough though. The transistor will only need about 0.6v to switch off...
Hi Fleck, sorry for the delay in getting back to you.
With LED's and 150Ώ to several outputs including Q14 (which is the one I've connected to the the 557's) All of them start flashing as soon as I apply power to the 12V rail.
As suggested, I've removed the 1k's from the bases of the 557's and have measured their base voltages. This has made no difference to the behaviour of the circuit, i.e. both the 555 and 4020 LED's flash as soon as power is applied and the relay, together with its LED, remain on.
  • Voltage at the 12V rail is 12.22V and the 557 base voltage is 11.32V; the base voltage shows this voltage as soon as power is applied to the circuit
  • Even at this voltage, the 557's have not switched off, the relay remains persistently on.
  • I've checked each 557 and the BC547 and they, according to my multimeter, are working fine.
  • I've gone over the circuit looking for mistakes in resistor values and shorts/bad connections but, as far as I can see, it's all okay.
It's puzzling because I can see why this circuit should work, even from my level of understanding! But that's as far as I get: I'll puzzle on...
Ah! yes, it does, when I bridge the 12V rail and the base of the 557's.
I guess that extra volt (12..22 - 11.34) is sufficient to turn off the 557's?
No shouldn't be any different.

I don't use PNPs very much but an NPN will be fully turned on with 0.8v on the base so you'd think a PNP would be fully off with anything more than 1v on the base.
Very puzzling as you say
I've searched youtube and found this link that seems to do the sort of thing I'm looking for.

this looks a bit beyond me at the moment!
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Yes that counter switches the PNP transistor off so why doesn't ours?
Check that the transistors are still ok.

I've arrived at work now so good luck, ill check back later