monostable timer

[EDDY99]

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

 

[ericgibbs]

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.?

 

[EDDY99]

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

 

[ericgibbs]

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.?

 

[EDDY99]

following so far

 

[ericgibbs]

following so far

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

The bottom two are my choice.

Any questions, ask.

 

[EDDY99]

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

 

[ericgibbs]

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.

 

[Boncuk]

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

 

[EDDY99]

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.

 

[ericgibbs]

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.

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

 

[EDDY99]

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

 

[Boncuk]

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

 

[EDDY99]

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

 

[Boncuk]

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

 

[Mikebits]

You might need to keep the 555 in reset until power is stable.

https://www.st.com/stonline/products/literature/an/11453.pdf

Also add some filtering on you Vcc.

 

[EDDY99]

Its probley because I dont know any better but the main panel switch has different functions with LEDS to tell me the state of the section of Railway track it controls. RED stop-GREEN-clear-AMBER timed-double AMBER timer working. The only way I could get this to work was by switching the timer on/off and make sure the reed switch connected to the the track was also inop. I know this is not the best soultion but I added bits to control the railway as I got ideas. I dont want to scrap the control panel and start again as too many hours have been spent on this already.

thanks for replying

eddy

 

[ericgibbs]

Its probley because I dont know any better but the main panel switch has different functions with LEDS to tell me the state of the section of Railway track it controls. RED stop-GREEN-clear-AMBER timed-double AMBER timer working. The only way I could get this to work was by switching the timer on/off and make sure the reed switch connected to the the track was also inop. I know this is not the best soultion but I added bits to control the railway as I got ideas. I dont want to scrap the control panel and start again as too many hours have been spent on this already.

thanks for replying

eddy

hi Eddy,
If thats the actual 555 circuit, I'll build one up on a project board, see if we can come up with a fix.
Most likely tomorrow, I am just finishing another job.

Are there any changes to the circuit you posted I should be aware of.?

 

[EDDY99]

Thanks very much Eric, the circuit board is the same (job lot for school projects) the only changes are, changing the buzzer for a relay and mounting the v-pot and switch on the control panel the longest leads used are about 2ft. The power supply is an off the shelf 3-12V 1A which I tried on 7.5-9-12V all with the same results.

many thanks Eddy

 

[ericgibbs]

Thanks very much Eric, the circuit board is the same (job lot for school projects) the only changes are, changing the buzzer for a relay and mounting the v-pot and switch on the control panel the longest leads used are about 2ft. The power supply is an off the shelf 3-12V 1A which I tried on 7.5-9-12V all with the same results.

many thanks Eddy

hi,
You must have a clamp diode across the relay coil.
The leads to the [2ft] switch should be twisted or screened.