Electronic circuits & projects forum - Need help with electronic circuits & projects?

[MSRobotics]

I found following circuits and one of them from the forum Courtesy "pebe" http://www.electronic-circuits-diagrams.com/forum/viewtopic.php?t=2640&highlight=push .

Circuit 1 is the 555 timer circuit with variable 1-10 minutes .

Circuit 2 is the 555 on / off switch circuit .

Here is what I want to do and would highly appreciate any help . I have attached both the diagrams in the image .

I want when some one triggers the Circuit 1 the time starts ( as set ) on completion of the time when the green light turns on it should starts the Circuit 2 and switch ON the relay of Circuit 2 ( without human pressing the button ) and doing so it should not effect the state of Circuit 1.

Now since the Circuit 2 is triggered by the Circuit 1 so If a person presses the push button on Circuit 2 it should reset ( relay should be released ) and should switch OFF . but again this process should not interrupt the Circuit 1 at all .

And more help i need I would like to use common power supply so I would like to know how can I change both to 12V or both to 9V

[pebe]

Both circuits can be made to work with different battery voltages.

For Circuit 1 to run on 12V, the value of the 470Ω resistor in series with green LED will pass about 18mA. That should be OK. I cannot say about the red LED because the resistance of the bleeper is unknown.

For Circuit 2 to run on 9V all you need to do is pick a relay with a lower coil voltage, because with a 9V supply the OP will go up to 7.5V max. A 9V relay will probably pull in OK at that but you would need to check the minimum pull-in voltage for the relay you intend to use.

Regarding the operation of Circuit 1. When you switch on, pin3 goes high lighting up the green LED for the timed period. At the end of that period, pin 3 goes low turning off the green LED and turning on the red LED and bleeper – not the green one as you said.

Circuit 2 is designed to sequentially turn on and off (ie. toggle) the relay, so I don’t see how it can be used with Circuit 1. Do you just want to use Circuit 2 to turn off Circuit 1 at the end of the timed period?

A better description of the sequences in your application would be helpful.

[pebe]

Both circuits can be made to work with different battery voltages.

For Circuit 1 to run on 12V, the value of the 470Ω resistor in series with green LED will pass about 18mA. That should be OK. I cannot say about the red LED because the resistance of the bleeper is unknown.

For Circuit 2 to run on 9V all you need to do is pick a relay with a lower coil voltage, because with a 9V supply the OP will go up to 7.5V max. A 9V relay will probably pull in OK at that but you would need to check the minimum pull-in voltage for the relay you intend to use.

Regarding the operation of Circuit 1. When you switch on, pin3 goes high lighting up the green LED for the timed period. At the end of that period, pin 3 goes low turning off the green LED and turning on the red LED and bleeper – not the green one as you said.

Circuit 2 is designed to sequentially turn on and off (ie. toggle) the relay, so I don’t see how it can be used with Circuit 1. Do you just want to use Circuit 2 to turn off Circuit 1 at the end of the timed period?

A better description of the sequences in your application would be helpful.

[MSRobotics]

Pebe , my mistake I said green it should have been red . when timer is started the green turns On and red turns Off and when time period comes to an end the green turns Off and red turns On . what I want is after the timer period finishes when red tuns ON same time it triggers the Circuit 2 and turns the Circuit2 On . where as turning off the Circuit 2 will be done by pressing the reset push switch on Circuit 2 by a person .

Also what is important is when person presses the reset push button on Circuit 2 in the processes should not cause a leak or surge to activate the Circuit 1 at all .

Also in Circuit 1 If i want a option to remove the buzzer can I do that ?

[pebe]

If you want to remove the buzzer, just replace it with a 470ohm resistor.

Like I said, Circuit 2 is designed to toggle the relay on-off-on-off, ad infinitum.

It can be amended so that it is triggered by Circuit 1 just once, then reset once with a push switch. Is that what you want?

[MSRobotics]

Yes thats exactly what I need .

Thanks

[pebe]

Here is the modified circuit. Connect C1 to your circuit 1 output.

[MSRobotics]

I built the circuit and it worked but I need help to make the 555 timer more reliable . It seems to me every time I start the timer the time period varies even though I didnt make any changes to the variable resistance . Also another issue I am facing is when I give power to the timer Circuit 1 it some times immediately goes to red or I would say starts with red light then I have to disconnect and reconnect few times to rest it so it starts with green light instead and turns red after time period comes to an end as per time set .
I dont want to complicated with complicated circuit so if possible would appreciate the help to remove existing circuits ..

[pebe]

I am not sure what you mean by your last sentence, but the following may help your problems with reliability.

The timing of the 555 is determined by the 220µF capacitor, the 100K resistor and the 1M pot. The only thing likely to change is the capacitor. But that would be over the long term – not over a short periiod .

Under normal circumstances, the sequence ends when the 220µF cap charges up and pin6 gets to 66% of V+. Pin7 then switches to 0V (battery negative) and discharges the capacitor. When you switch off, the 220µf will then absorb the charge from the 100nF cap and so discharge it ready for the next time.

I think the result you are getting is if you switch off the supply before the timer has finished its timing sequence. There is then nothing to discharge the 220µF cap and the next timing period will be too short. Also, the 100nF capacitor will take up the same voltage as the 220µF cap and if that is higher than 33% of V+ the 555 will not trigger next time and the red LED will come on.

You can overcome the problem by ensuring that both capacitors discharge when you switch off. Do it this way.

Connect a 4K7 resistor between Pins 8 and 1 of the 555.
Connect a 1N4148 or similar diode from 555 pin7 to pin8 (diode cathode to pin8).

[MSRobotics]

one of the problem which is started earlier of timer circuit starting in triggered mode got solved as rightly explained by you of switch of before time finishes .

But my other problem is still there of no reliability . I felt that my timer is not stable so I took a stopwatch and started the timer withou changing time period ( no changes to variable resistance ) . first time timer completed at 54 seconds , second time it did at 49 seconds third time it did at 38 seconds fourth time it did as 42 seconds and first time it did at 29 seconds where as I didnt not make any changes to any thing at all . so it seems timer is not reliable .

Is there a way I can make it reliable I need a reliable accurate 1-15minutes timer on 555 . are there any changes to make this same circuit more reliable ??

[pebe]

The 555 is a rugged IC that gives very stable timing periods, but like everything else, it is liable to go faulty. In your case where your timings are varying, a faulty 555 is unlikely. That said, if you have a spare 555 then replace it to prove the point.

The timing period in seconds is equal to 1.1 x C x R, and that is the time taken for the capacitor C to charge via resistor R, from 0V to 2/3 V+ (the 2/3 V+ is determined by internal resistors in the 555 and junction of those resistors is brought out to pin5).

So one of these parameters is wrong

1. The capacitor should start charging from 0V
2. The end-point, 2/3 V+, is not correct
3. R is intermittent in value
4. C is intermittent in value

First, thoroughly check out your connections. If they are OK then check out these points in turn. Run the timer several times, checking your results after each timed period.

1. Measure the voltage across the 220µF capacitor after timing. It should be almost zero (if it varies then pin7 is not discharging properly and the 555 is faulty).
2. Measure pin5. If you are using 12V supply then it should read 8V
3. The pot may have an intermittent wiper (I think that’s the most likely fault). Try shorting out the pot, leaving just the 100K in circuit.
4. Unlikely to be at fault, but try replacing the 220µF capacitor

Hopefully, you can find the fault from those checks.