AC One-shot/Timer?

Velocit

I have a couple projects that could use 110VAC power for a specified amount of time after being triggered; like a one-shot/monostable multivibrator. Does anyone know of a non-electromechanical way of achieving such a thing? I entertained the idea of using a low RPM synchronous motor with a cam and switch, but I've determined that's not going to work for at least one of my projects. I'm also trying to minimize the part count and cost, so I'm trying really hard to avoid any DC conversion (well, I just don't want any bulky transformers. I'm open to half-wave rectification.). Am I asking the impossible?

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-Paul

Leon

An AC SSR with a small MCU for the timing function? You'd need a mains supply for the MCU.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Mike Green

I couldn't give you a specific circuit, but a solid state switch or your own triac-based relay would work. There are all sorts of ways to set up a trigger that will stay on for a brief time after being triggered through an optocoupler (for isolation) using an RC delay. A 555 timer would work. If you want to use a contact closure to start the interval, you could use a power triac with an optoisolated triac as a trigger. You'd need a small transformer-based power supply for the 555 timer (on the isolated side now). It would probably be easiest to use a tiny wall-wart to supply the power to the timer portion.

If you want something very straightforward, use a SSR with a BS1 as the controller and a little 5V wall-wart to power it. There are lots of 5V wall-warts on the market now for powering USB-based devices.

Velocit

Thank you, those are both very good suggestions. I'm really trying to avoid any conversion to DC, however. I've worked with simple RC networks and 555's before, but I'm curious as to whether or not there's a way to accomplish timing with no DC components at all. Like many of us on the Parallax forums, most of my experience is digital and I rarely work with voltages above 24VDC -- so I just don't know what is possible when working with 110VAC.

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-Paul

kwinn

Pretty hard to do timing without using dc components. Low voltage DC at low currents is not that hard to get without a transformer. You can use a capacitor, bridge rectifier, and zener to produce a dc voltage that would run a timer chip or micro for a couple of dollars and use that to trigger a triac or ssr.

PJAllen

Make sure that you use a fuse and keep your Hot and Neutral right and you can go transformerless: "rectify the wall" --·which is always a hoot.· Put on your thinking cap and you can employ impedance, R & C (R·& -j), for hours of fun and excitement, too.

stamptrol

Many general purpose industrial timers are designed to be delay-on, delay-off, or single shot. Have a look on e-bay for Omron timers.

A second cheap method would be to search for a Seimens "LOGO" programmable relay on e-bay, etc. No programming software required, available in relay outputs, operates on 120 - 240 VAC and probably available for $20 or less.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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