Basic 555 timer

[Stonesman]

Hi Folks,

My 1st post!

I'm a cctv installer by trade. I was asked to install a video door entry system recently.

The system gives a 1 second pulse when the operator presses the 'open door' button.

1 sec isn't long enough to allow a visitor to enter through the main door. To extend this time I built a maplin 555 timer kit that provides a delayed output to a pulsed input.

The circuit works fine on the bench with no outputs connected. As soon as a COM (+12V) and a NO (output to door latch) is connected the relay stutters.

I find this odd that the relay output effects the circuits stability - am I missing something?

Cheers,

Stones.

 

[KrisBlueNZ]

Hi and welcome to Electronics Point

Is the door latch circuit powered from the same power supply that is powering the 555 board? Is the power supply properly rated?

If the power supply is under-rated, when the relay closes and connects the power supply to the door latch, the extra current could cause the power supply voltage to drop. This will affect the 555 board and could be the reason for the problem.

To test for this problem, measure the power supply with a voltmeter when the problem is occurring.

 

[Ehsan]

Yes, Kris is right. To troubleshoot this try to have two complete separate power supplies, one for the 555 timer and pulse circuit behind it, and one for the circuit after the relay.

 

[davenn]

of course there is also the obvious possibility that the output of the 555 isn't able to hold the latch solenoid
hence why it chatters
you may need the 555 to drive a transistor that will then switch the latch

Dave

 

[KrisBlueNZ]

of course there is also the obvious possibility that the output of the 555 isn't able to hold the latch solenoid

He mentioned a relay; I assume that's on the 555 board between the 555 and the output that drives the door latch solenoid.

 

[Stonesman]

Great replies, thanks.

I did think about about the psu being under-rated (latch is rated at 400mA, psu is 1000mA so should be OK).

To make sure, I used a separate supply to the timer PCB. In therory making the timer and door latch completely separate - but the problem was still present!...

The timer relay is rated at 10A so should be able to cope - I'm stuck!

Mmmm...

 

[KrisBlueNZ]

Is the door latch solenoid polarised? It may have a diode built in to absorb the back EMF. Try connecting it the other way round.

Can you post a link to the timer kit you built? And a photo of each side?

 

[Stonesman]

Latch fires fine when power is manually applied with same polarity - although will try reversing when switching through timer.

Kit is here...
http://www.maplin.co.uk/p/555-monostable-switch-kit-n32fl

Pics attached...

 

[KrisBlueNZ]

Did the kit include a schematic diagram? If so, could you post that as well please?

 

[Stonesman]

No, it didn't actually - that was one of the negative feedback ratings about the kit!

Thanks for responding.

 

[KrisBlueNZ]

Pfft. That's not very good. I'll try to figure it out from the photographs then.

 

[KrisBlueNZ]

OK, that relay is a Multicomp HRS4-S-DC12V with a coil power of 720 mW which corresponds to a current of 60 mA. Its coil is connected between the positive rail and the 555's output, with a diode (D2) across it. The LED (with its series resistor) is also connected across the relay coil. This is all fine.

I notice there's no decoupling capacitor on the 555. You can try adding one - a ceramic capacitor between 0.1 µF and 1 µF is fine. Connect it between pins 1 and 8 of the 555 on the underside of the board, like this. Keep the wires as short and direct as possible.


Here are some suitable caps from Maplin, in order from best to worst..

http://www.maplin.co.uk/p/decoupling-ceramic-disc-022uf-capacitor-jl01b 0.22 µF disc ceramic
http://www.maplin.co.uk/p/multilayer-metallised-polyester-film-470nf-capacitor-ds80b 0.47 µF film
http://www.maplin.co.uk/p/decoupling-ceramic-disc-01uf-capacitor-n43cj 0.1 µF disc ceramic
http://www.maplin.co.uk/p/decoupling-ceramic-disc-01uf-capacitor-bx03d 0.1 µF disc ceramic

 

[Stonesman]

Brilliant, thank you for your hard work.

I'll get to Maplin and get the capacitor.

Will let you know.

Thanks again!