NE555 Timer circuit Problem

Thread Starter

Jtptom

Joined Aug 30, 2018
6
Hi All,
I am looking for help with attached circuit I had it working on bench, but then started to get random things happen it seems that one timer hinders the other, I have simulated it in circuit wizard and I can not get it to work at all.
The idea is to activate sw1, the relay is to operate for an adjustable time then deactivate the relay.
any help would be appreciated,
I was intending to use 556 but wanted it working on the 555 first.
I do apologize if I have posted in wrong area but I was not sure.
thank you.



555circuit.png
 

AnalogKid

Joined Aug 1, 2013
9,249
I can not get it to work at all.
Not very useful. Please provide more detailed descriptions of its behavior. When I do this, it does that ...

1. Add decoupling capacitors across each 555 from the Vcc pin to the GND pin. 0.1 uF ceramic or larger, shortest possible leads.

2. Replace C2 and C5 with 10 nF or larger. 100 pF is too small too do any good.

3. What is the relay coil resistance? And, is it rated for continuous operation at 15 V? Depending on the current requirement, it might be possible to eliminate Q1 and Q3, R3 and R10.

4. As a circuit tune-up, consider changing C1, C2, C4, and C5 to the same capacitor type as used to decouple the chips. Parts commonality is a good thing.

Describe in more detail how the circuit is supposed to work. My guess from reading the schematic is that when SW1 changes position, the circuit for that side activates. The relay is activated for approx. 16 seconds. Is this a good guess?

Note - if you connect the armature of SW1 to GND instead of +15 V, you can eliminate Q2, Q4, R5, and R9.

AND - just because my brain is running - - - IF you use SW1 to switch power to the two 555 circuits rather than just a trigger signal, four more parts go away.

ak
 
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Thread Starter

Jtptom

Joined Aug 30, 2018
6
Not very useful. Please provide more detailed descriptions of its behavior. When I do this, it does that ...

3. What is the relay coil resistance? And, is it rated for continuous operation at 15 V? Depending on the current requirement, it might be possible to eliminate Q1 and Q3, R3 and R10.

Describe in more detail how the circuit is supposed to work. My guess from reading the schematic is that when SW1 changes position, the circuit for that side activates. The relay is activated for approx. 16 seconds. Is this a good guess?

AND - just because my brain is running - - - IF you use SW1 to switch power to the two 555 circuits rather than just a trigger signal, four more parts go away.

ak
Thanks for reply,
At the moment as soon as power is turned on the circuit with the switch connected fires the relay for around 16secs then relay turns off after that nothing happens when operating the switch.

I do not know the resistance of the relay coil, the idea is to try and keep it as low current as possible, the relay is not rated for continuous operation.

Your educated guess is correct, operation of the switch activates the side of the circuit it is connected to, relay activates for an adjustable time then the relay shuts off, if switch is operated during this time it shuts off that circuit and activates the other.

I am using trigger, as I thought powering the chip each time would affect its operation, and I hope to convert to 556 at later stage.
 

sghioto

Joined Dec 31, 2017
2,363
I am using trigger, as I thought powering the chip each time would affect its operation,
In the circuit below SW1 will have priority over which circuit is activated. Meaning only one timer can be activated at a time.
1613949734776.png
 
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dl324

Joined Mar 30, 2015
12,871
(Hinder) (Affect) sorry if did understand
Does triggering one timer cause the other timer to trigger? Does triggering one prevent the other from triggering?

We can't see what you've seen, so can only go by your words. Hinder can have a lot of meanings, but it's not usually used to describe circuit problems.

If you didn't have adequate supply decoupling, relays being switched can put spikes on the power lines. Sometimes no amount of decoupling will help.

What loads are being switched by the relays? Could the relays be replaced with transistors for high or low side switching? Have you considered solid state relays?
 

AnalogKid

Joined Aug 1, 2013
9,249
In the circuit below SW1 will have priority over which circuit is activated.
I almost posted that circuit. Problem is that there is no discharge path for C3 when the switch is flipped to the other circuit. Consider adding a 10K resistor between pins 1 and 4 to reduce the discharge time to approx 3 s.

ak
 

sghioto

Joined Dec 31, 2017
2,363
C3 discharges through the 555 and D1.
No 10K resistor needed. Internal NE555 has three 5K resistors in series to bias the comparators.
 

AnalogKid

Joined Aug 1, 2013
9,249
C3 discharges through the 555 and D1.
No 10K resistor needed. Internal NE555 has three 5K resistors in series to bias the comparators.
Good point; forgot that they are directly connected to pins, rather than through an internal current source circuit or something else.

ak
 

Thread Starter

Jtptom

Joined Aug 30, 2018
6
Is this what it is doing now or is that what you want it to do?
What is happening now is, as soon as power is turned on the circuit with the switch connected fires the relay for around 16secs then relay turns off after that nothing happens when operating the switch.

What I would like to happen is, operation of the switch activates the side of the circuit it is connected to, relay activates for an adjustable time then the relay shuts off, if switch is operated during this time it shuts off that circuit and activates the other.

Thanks for your help.
 
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