momentary switch

[andrew]

I'm looking for the simplest monostable to provide a short pulse to momentarily close a contact on our electric gate at work. Normally it's opened with remote control but for visitors during office hours is was equipped with a bell push for entry and exit, this being cheaper than induction loops etc.

Some wag has found that if the outside button is depressed and held down the gate cycles through open to closed and then will not accept input from the other button or the remotes.

I'm after a simple way to have the bell pushes close the contacts momentarily and then release even if the bell push is held down.

I've got a Crouzet MUR1 that I can get to do the job but it's about 30 quid and I wonder about something simpler?

AJH

[Bill Wright]

Make the bellpush live, then it will only be held down momentarily.

Bill

[The Natural Philosopher]

simplest is a resistor a capacitor and a pair of CMOS gates.

You might need a buffer with some current to drive anything of any use.

SEE

"NOR gate monostable".

[BartC]

How about just a large capacitor? Normally connected to a power supply, pressing the button (a changeover switch) connects one end to a relay instead (the other end is common). Value and relay (and perhaps series resistor) chosen to provide a long enough pulse.

This doesn't stop someone pressing repeatedly, but neither does a monostable, unless you have a second monostable that stops you operating the switch again for a longer period.

[Steve Walker]

He might even get away with a dc supply, the pushbutton, a blocking capacitor, a drain resistor and a relay. Simple and bombproof.

SteveW

[The Natural Philosopher]

I think the condition to stop is jamming the button down.

You are right that a capacitor with a leaky resistor across it that is connected in series with a relay wont hold the relay in for ever. If te resistor is relatively large you will have to wait a fair time before it works again, too.

[andrew]

I was considering just this and rummaging through my failed projects box I have a Hamelin HE3312A 12V reed relay donated by a denizen of this group.

It seems to have a coil resistance of 500 Ohm and I think holding on the contacts for 100millisec would trigger the gate. It latches at 3.75V and the initial current will be 24mA but what size of capacitor? What size resistor to leak the capacitor to ground? I'd guess it needs to be an order of magnitude bigger than the coil resistance?

AJH

[Steve Walker]

You've mentioned 3.75V, but it's not clear whether that is the pull-in or drop-out voltage, so I've used it for both and you'll have to adjust values where necessary.

I've totally forgotten my capacitor charge equations so I took a look at

think that with a 12V supply, 500 ohm coil and a 470 microfarad capacitor, the current will will fall to 7.5 mA (3.75V through 500ohm) in around 0.27 seconds.

Decrease the capacitor to 220 microfarad and you'll get around .13 seconds.

The drain resistor can be worked out using

'd go for a large value to delay the time before the button can be operated again, just in case repeated pulses cause any problems.

Around 390K would discharge to 8.25V in around 30 seconds leaving 3.75V (12V-8.25V) available again, but would give only a very short pulse if the button were pressed again. It'll take a few minutes to discharge enough for a virtually full length pulse though, so you may want to use a smaller value.

I hope I've got all that right - it's a very long time since I've done this.

SteveW

[andrew]

This should be the pull in voltage, spec sheet says it has to release by

0.5V

30 second cycle will be fine, it takes 20 seconds to fully open the gate then it waits 10 seconds before closing, it can be triggered any time it is clsing to start the opening cycle again.

Thanks Steve I have set up all the bits for a trial ( microswitch in series with solenoid coil and a bulb across the normally open contacts with 9V pp9 for power), just searching for capacitor and resistor.

AJH

[Hugh Jampton]

Or a NE555 timer. Trigger with a capacitor and fit a pot to adjust the pulse length.