Timer circuit: Switching transistor on Low state?

Thread Starter

mjelic

Joined Mar 11, 2008
21
Hi Mik,

Only finalised it this morning, and was going to report back but you beat me to it. :)

Did it work?
Yes and no. :)

Yes, your suggestion worked perfectly in terms of making the relay go on when the Q14 went low (the reverse of what it did originally) and so that bit worked perfectly.

But... The 555 would refuse to reset and count at all once the relay was on. After a LOT of testing of voltages, and soldering on LEDs to see the states of things and clocks running or not, I figured out the problem. When the relay is switched on, the line to the 555 is pulled low and thus STOPS the clock.

The answer was as S0ren put up... I had to cut the track to the 555 from the orginal NPN and tie it to the collector of your new Q2. I see he suggests a 22k resistor, but the 33k I found seems to be working OK. *shrug*

So, all up, I'd like to thank you heaps for your help. Your quick responses were very much appreciated. Job is now done, but tinkering with all this seems to have ignited my love of electronics again, so I want to try making this circuit even simpler, by using just a 555 and a relay. (No 4020 counter as I really only need the timing to be between a minute to 10 mins max.

You'll probably see me on here again soon. :)

Cheers,
Mark
 

Thread Starter

mjelic

Joined Mar 11, 2008
21
Søren;60108 said:
This will:
Yes, that is what I have ended up with right now, except the 22k resistor you have is the 33k the Mik suggested, and I still have the diode from his suggestion in place. (I have to admit that I'm not sure why the diode is needed, but hey, it works :))

But, now that we have established that the 555 needs a high to count, and in the original circuit the 555 reset would be pulled low when the relay was switch on (and thus stopping the count and keeping the relay engergised) surely now the original suggestion by Mik (using a PNP tranny) would work, and be a simpler solution.

See my hacked up circuit diagram attached. (NOTE: I have no idea what resistor values to use - just went with what was in the original.)

Not that I am going to tinker with it any more as it is now working. :) Just want to understand how this all works and see if what I think should work makes sense to you gurus. :)

Cheers,
Mark
 

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Søren

Joined Sep 2, 2006
472
Hi,

Yes, that is what I have ended up with right now, except the 22k resistor you have is the 33k the Mik suggested, and I still have the diode from his suggestion in place. (I have to admit that I'm not sure why the diode is needed, but hey, it works :))
It will work with a vide range of resistor values. It should just be large enough to avoid unneded current consumption and small enough to be able to deliver the needed base current for the relay driving transistor.

Not sure which diode that would be, but even the D1 from the original circuit seems to be thrown in for no reason at all - it's not needed.


See my hacked up circuit diagram attached. (NOTE: I have no idea what resistor values to use - just went with what was in the original.)

Not that I am going to tinker with it any more as it is now working. :) Just want to understand how this all works and see if what I think should work makes sense to you gurus. :)
Seems OK.



I want to try making this circuit even simpler, by using just a 555 and a relay. (No 4020 counter as I really only need the timing to be between a minute to 10 mins max.
This should do:
 

Thread Starter

mjelic

Joined Mar 11, 2008
21
Søren;60337 said:
Not sure which diode that would be, but even the D1 from the original circuit seems to be thrown in for no reason at all - it's not needed.
The kit instructions said D1 was used to discharge the capacitor C3 when the power was turned off. Frankly I didn't think that would work and didn't think it was all that important to discharge anyway!

Søren;60337 said:
Seems OK.
Yes, I thought so too. If I find I have some free time, I might try it out just to see if my theory is right. :) I purchased some bread-board the other day, and the ICs in the circuit are in sockets, so I should be able to pull them out and tinker with them easily enough. :)

Søren;60337 said:
This should do:
Thanks for the circuit and the formula. R3 shows "1..10K". Does that mean it can be any value between 1 to 10K?

I have a number of trim pots at hand (10K, 20K, 100K) so I'll try and get a capacitor value that gives me a variable time delay between 1 and 10 minutes.

Thanks again.
Mark
 

Thread Starter

mjelic

Joined Mar 11, 2008
21
Søren;60337 said:
Hi,

This should do:
Hi Soeren,

Just wanted to report back that the circuit you suggested works perfectly. :)

I ended up using a 470uF capacitor, and a 100K resistor in series with a 220K trim-pot to give me the timing range of about 10 seconds to several minutes, which is exactly the range I needed.

One question: Can a 555 handle driving a relay directly or should it really drive a transistor that switches the relay on or off? If the latter, can you make a suggestion as to what transistor to use, and any associated current limiting resistors required?

Thanks for your help so far! :) I have it all working on a tidy little piece of bread-board, driving an LED at present. I know, very simple stuff, but I'm happy to be dabbling in electronics again. :)

Mark
 

Søren

Joined Sep 2, 2006
472
Hi Mjelic,


One question: Can a 555 handle driving a relay directly or should it really drive a transistor that switches the relay on or off? If the latter, can you make a suggestion as to what transistor to use, and any associated current limiting resistors required?
The regular (i.e. non-CMOS) 555, when run from 12V, can handle relays with a coil resistance of 60 Ohm or larger.
I haven't ever seen an automotive relay with such a low coil resistance - 150..200 Ohm is quite common for a 40A automotive relay.

To test the relays you have, just measure the current they take from a 12V supply - as long as they need less than 200 mA (the max. current of the 555), you are in the green.


Thanks for your help so far! :) I have it all working on a tidy little piece of bread-board, driving an LED at present. I know, very simple stuff, but I'm happy to be dabbling in electronics again. :)
You are welcome and yes, it beats hanging out on street corners :D


R3 shows "1..10K". Does that mean it can be any value between 1 to 10K?
Exactly, but I guess you found out :)
 

Thread Starter

mjelic

Joined Mar 11, 2008
21
Søren;63903 said:
The regular (i.e. non-CMOS) 555, when run from 12V, can handle relays with a coil resistance of 60 Ohm or larger.
I haven't ever seen an automotive relay with such a low coil resistance - 150..200 Ohm is quite common for a 40A automotive relay.
Hi S0ren,

Well I found a number of small 12v relays that had coil resistance's varying from 300 ohm to 700 ohms (settling on a DPDT type that has a coil resistance of 400), but I still ended up using a BC338 to drive it, which works fine.

But, I have another question for you (or any members willing to chime in) on the mysterious workings of the 555.

As I said, I have it working perfectly in terms of being able to vary the time the 555 takes keeping the output at pin 3 high, from 8 secs to several minutes, before going low. But the original circuit (from the timer kit, with the 4020 ripple counter) had a feature that I just discovered was missing from your very simple 555 circuit that I am now tinkering with: A "reset" or more accurately, a "restart" function. What I mean is that when the start button is pressed, the timing starts, but if you press it again, the full interval should be started from that point again. (It does not do that with the existing 555 circuit. Pressing the button a second time, during the timing cycle, does nothing.)

For example, on the Flexitimer Kit, if the interval is 1 minute, and I re-press the start button even seconds away from the end of the interval, the button would reset the 4020 counter and the counting sequence would start again, taking one minute from the time you re-pressed the start button.

So, the question I have is: Is there a "restart" capability with the 555? Once the timer has been triggered and is going through the cycle, using the same button (although it could be on a different throw of the same switch, if required) can I re-start the timing cycle without interrupting the high output state?

(Sorry for making this so wordy: Hard to describe what exactly I am trying to achieve here.)

Thanks, as always, in anticipation of your assistance.

Regards,
Mark
 
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