Need advice with a 556 timing circuit

Thread Starter

Otaku

Joined Nov 19, 2008
128
Thanks, Bill!

A thought...
The timing errors (timer A re-triggering) always occur at the transition from timer A to timer B. Timer B is triggered by the relay controlled by timer A. Timer B controls another relay that opens the connection from the PIR's output to the base resistor of a Darlington, which disables the ability of the PIR to trigger timer A until timer B runs out.
Is there a chance that the errors are caused by the timer B relay switching? If so, I can remove the relay and substitute a PNP transistor, using the high output from timer B to open the PIR connection.
The existing traces on the board will work for this substitution. The only thing I don't know is what the value of the PNP base resistor should be. I would be using a 2N3906 PNP. The voltage to the base resistor would be ~10.3VDC (12VDC - ~1.7VDC from the 556). I'm not certain of the current delivered by the PIR output, but the voltage is a range from ~2VDC to 3.2VDC. The 2N3906 datasheet states the collector current limit is 200mA - does the PIR output current need to be less than this value for this to work? Is there a more suitable PNP for this application (higher collector current)? What do you think?
 

Bernard

Joined Aug 7, 2008
5,784
This is a suggestion for replacing Ry B. Replace darlington with lower gain 2N3904. replace 100 Ω with 1k Ω, add 4.7kΩ base to ground[ repeat suggestion] & add another 2N3904 to shunt PIR switch 2N3904 base to ground. When B is high, PIR input is disabled, when low, normal PIR operation.
 

Attachments

Thread Starter

Otaku

Joined Nov 19, 2008
128
Thanks, Bernard. I'm curious as to why the Darlington would need to be replaced. If I use a PNP in place of Ry B, wouldn't that achieve the same thing? I think I have some 2N3904's laying around - I'll try your suggestion tomorrow.
 

Bernard

Joined Aug 7, 2008
5,784
In the origional schematic the darlington was left with a high impedance base lead floating, like an antenna looking for stray signals, is why 4.7k was suggested to tame noise input.
 

Thread Starter

Otaku

Joined Nov 19, 2008
128
Whew! I think we have the solution.
Bernard, I used your suggestion regarding changing the pull-up resistor on the Darlington to a 4.7K, and connected the NO pin of Ry B to ground to eliminate the floating base lead. Running AC power through the on-board relay (Ry A) still throws re-triggering errors, but when I use an off-board relay to switch the AC, all's well.
A couple of caveats:
The AC motor needs to be a few feet away from the circuit board. Same goes for the off-board relay. Apparently AC induction loads create a fairly robust interference field (is that the correct term?) and keeping a couple of feet between the AC sources (motor and relay) prevents re-triggering. I was doing the testing in a fairly confined space, so all the components were within ~12" of each other. It looks this issue is solved, pending some further tests.
I'm wondering if mounting the off-board relay in a small metal project box would allow me to place it closer to the circuit board? Probably not, but it's worth asking...

Many thanks for all the input and advice - much appreciated!
 

Wendy

Joined Mar 24, 2008
23,415
OK, I've finished your original schematic. I need to add whatever changes you may have done. Then we can go through the logic of it.



I've also added designations, this is a very good habit to get into. It allows you to discuss components with no ambiguity whatsoever.

Do you want me to label the components on your bread board layout and post them?

Looking at it, you want U1b to trigger with the PIR output goes high?

When U1b output goes low, U1a is triggered. This disables the U1b for a set duration. Correct?

Whew! I think we have the solution.
Bernard, I used your suggestion regarding changing the pull-up resistor on the Darlington to a 4.7K, and connected the NO pin of Ry B to ground to eliminate the floating base lead. Running AC power through the on-board relay (Ry A) still throws re-triggering errors, but when I use an off-board relay to switch the AC, all's well.
A couple of caveats:
The AC motor needs to be a few feet away from the circuit board. Same goes for the off-board relay. Apparently AC induction loads create a fairly robust interference field (is that the correct term?) and keeping a couple of feet between the AC sources (motor and relay) prevents re-triggering. I was doing the testing in a fairly confined space, so all the components were within ~12" of each other. It looks this issue is solved, pending some further tests.
I'm wondering if mounting the off-board relay in a small metal project box would allow me to place it closer to the circuit board? Probably not, but it's worth asking...

Many thanks for all the input and advice - much appreciated!
There should be a pull down resistor on the input of the darlington, not a pull up. It can be 10KΩ, the exact value isn't important (but it should be high to avoid a voltage drop because of R9).
 

Attachments

Last edited:

Wendy

Joined Mar 24, 2008
23,415
OK, here are the changes I recommend in your schematic to make it more stable...



Try it and see how it works out.

Couple of side comments, overall the design is pretty good. I'm always uncomfortable driving relays with 555's, but if the draw is under 100ma then it's OK.

You don't really need CR1 or CR3 (CR2 and CR4 are absolutely necessary).

I also agree with replacing Q1 with something like a 2N2222A. A single transistor turns on hard, while a darlington (which has high gain that isn't needed for digital use) drops 0.6 V across the collector emitter when it is fully on.

R9 could also be as high as 10KΩ and still work. I'd go with 1KΩ.
 

Attachments

Last edited:

Thread Starter

Otaku

Joined Nov 19, 2008
128
Thanks very much, Bill. This is waay easier to understand than my layout drawing. I'll build this out on a perf board and see how it handles the AC switching.
 

Bernard

Joined Aug 7, 2008
5,784
Bill- are we using a different LED now, origional one was common cathode with R10 returned to ground. Maybe I started this error.
Also , I could not find pads for the LED on PC Board.
 

Wendy

Joined Mar 24, 2008
23,415
Opps. Fixed.



It really doesn't matter if the LED is on the PCB or not, its part of the schematic. I figure it is on the front panel.

The current transistor (the darlington) will work as it. Basically it is overkill for the job.

****************************************************************

BTW, these are not retriggerable monostables. They will ignore any pulses they get while timing.
 

Attachments

Last edited:
Top