Hi, can anyone pse help a novice.
I am building a model railway control panel and I am stuck. On my layout I have station stops activated by the loco triggering a reed switch,
this works fine. I decided to add a monostable timer to automate the length of time the loco is stopped. A DPDT C/O MIN TOGGLE SWITCH controls this
ON manual stop - OFF no stopping - ON timed stop. I made a tester and all worked fine but I now have a problem that some of the timers
when switched on go straight into timed mode instead of waiting for the reed switch to trigger it. Can anyone suggest a retro fit to cure this ??
I ask for a retro fit because I have fitted 40 of these timers already and about 50% have this problem.

Thanks eddy

 

hi eddy,

How are you debouncing the toggle switch.?

Also what type of monostable.?

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Its a basic 555 monostable, "debouncing" ? as u can tell I am a complete newbie to electronics, but willing to listen and learn.

 

hi,
The problem is that with most mechanical switches you dont get a single contact action between the contacts.
They bounce on/off when closing for a few millisecs, as the logic is so fast its senses all these on/offs as signals. These bounce signals can cause your circuits to do unexepected events.

So between the switch contacts and the 555 you need a simple resistor and capacitor circuit.

If the switches are really 'bouncy' you may need an additional ic, eg a 40106 or 74HC14 Schmitt trigger.

Do you follow OK.?

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following so far

 

hi,
This shows some options, there are many.!

The bottom two are my choice.

Any questions, ask.

Attached Thumbnails

 

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Thanks very much, should be able to manage that, do u have any values for the V & R in the first one (top left) power supply is 12DC

 

hi,
Look at these two options.

I prefer the Schmitt.

EDIT: as you are using 12V, the 74HC14 should be replaced by a HEF40106.

Attached Thumbnails

 

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Eric " Good enough is Perfect "
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Third option ...

That one doesn't care if the switch is debounced since it's non-retriggerable until it times out.

Triggering is done with a positive going pulse edge.

Hans

Attached Thumbnails

 

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Thanks very much for the quick replies, I have tried a couple of the suggested circuits but anything that alters the power to the switch has a knock on effect of making the other functions of the switch inop. The switch is used to trigger relays for point motors/coloured lights etc. When switched to timed all I am doing is powering up the monostable ready to be triggered by the loco reed switch, if I turn the v-pot to zero it behaves as normal but if the pot is turned up it goes straight into timed mode, after the set time it does reset it self and acts normal.

Attached Thumbnails

   

 

hi eddy,
Is this the actual circuit you are using.?

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Eric " Good enough is Perfect "
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The circuit is the same, made a couple of changes, removed the power in switch so it is powered from the panel switch, the v-pot is also mounted on the panel and the buzzer has been replaced with a relay

 

Hi Eddy,

that circuit is retriggerable. I didn't detect any decoupling capacitor either. Also connect pin5 (Control Voltage (CV)) via 10nF to ground.

Hans

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Hi, i dont have any trouble retriggering the timer the prob is it triggers when switched on, which i want to get rid of.

 

So why do you disconnect power then? Every timer circuit produces a short output pulse when energized.

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