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Indicator LED for valve controller
calzap:
I have a controller for 24 VAC irrigation valves in a pump house and would like to have a flashing LED on an outside the pump house wall that indicates when at least one valve has power. To be clear, there will be only one LED, not one for each valve. Each valve already has an LED, but the valves are located in boxes or cabinets 5-100 m away from the pump house.
There is plenty of 24 VAC and 9 VDC power available. The 9 VDC is used inside the pump house to power indicator LEDs which get switched via DPDT relays or DPDT switches. Other pole of the relays and switches controls 24 VAC to valves.
Simplest and cheapest solution I have devised is to have a small SPST (or SPDT) relay for each valve control "module" ... relays to have 24 VAC coil. Cost of relays is less than US$ 4. See the pic.
Other ideas?
Mike in California
Edit: Yes, I have considered a rectifier for each valve unit to send power to the LED and may go that way.
Ian.M:
We can discuss circuit details later once you've picked a way to go:
A diode for each AC valve output, cathodes wire ORed together would provide half-wave rectified DC when any valve is active. Smooth it and regulate down to an appropriate voltage for a commercially available exterior LED security strobe.
A bunch of AC input optocouplers + a dropper resistor for each to monitor all the AC outputs, with their photo transistor outputs wired in parallel . . . .
A current sensor on the 24V feed to the switches and relays - if there's negligible current all are off, if there's one or more on the current will be detectable. Add a comparator for thresholding and low pass filter its output to eliminate line frequency 'chatter' to drive a transistor to control a LED strobe.
calzap:
Ian,
Thanks for your response. I've been leaning toward electromechanical relays because it's hard to beat the isolation and environmental toughness they provide. However, I did consider the diode/rectifier solution, which is made easier by the fact that the valves share a common 24 VAC line. See the first pic. It's hard to beat this solution for price. Do you see any downside in it?
I thought of optocouplers/SSRs but decided they wouldn't be easier or cheaper than electromechanical relays.
Didn't think of current sensing. The 24 VAC is used for several purposes in the pump house. And there is are several multiwire cables leaving for the valves. Not sure there is a single spot where there is just one wire carrying the common 24 VAC to the valves, but this could be created. Still doubt I would go this route.
Second pic is the LED circuit I've used for years, powered by 24 VAC and used at the valves. No smoothing has been necessary (but haven't tried it with flashing LEDs). No electrical component failures. Biggest problem was encapsulation. Tried clear heat-shrink, but it didn't hold up. Went to polycarbonate tubing and haven't had a problem.
Mike in California
Ian.M:
Yes, that would work for plain LEDs if you don't mind the 60Hz flicker. However if you want to be able to use an off-the-shelf LED strobe intended for exterior mounting or even a self-flashing LED, you'll need to smooth the half-wave rectified DC and limit it to approx. 12V.
The circuit lineup after the diodes would probably be something like a 15R, 3W wirewound resistor to limit the inrush current, charging a 50V 1000uF electrolytic capacitor, feeding a LM317 regulator with a 120R upper feedback divider resistor and a 1K2 lower resistor for approx 13.5V out, near enough the same as the float voltage of a 12V SLA battery, which most security system accessories (e.g. a LED strobe) expect as their supply voltage, and with 100uF,16V decoupling on its output. That would be good for up to 200mA to power the strobe. If it draws over 100mA the LM317 will need a small heatsink. The diodes should be min. 1A, 100V. N.B. The 1000uF capacitor *MUST have a 1/3A or greater ripple current rating for 200mA load current - if nessercery use a higher value capacitor or two in parallel to get a high enough ripple current rating. If you need more than 200mA, decrease R1 to 10R, replace the regulator with a nominal 3A Dc-Dc buck module from EBAY set to 13.5V and omit C2, R2, R3. However its not a good circuit for high load currents - if you need that you'd be better off minimally smoothing the voltage from the combined diodes and using it to switch a SSR controlling a separate 12V PSU.
calzap:
Ian,
Again thanks. You're very helpful. So far I've used self-flashing LEDs. A minor hobby is collecting as many different 5 mm types as I can find ... 14 distinct types so far. However, I've always powered these with DC. For AC powering, I've only used non-flashing LEDs powered with the simple circuit in my previous post.
I have plenty of self-flashers, so I'll try one AC-powered with the simple circuit. Should be able to do it on the bench next week. Then if it dies (likely), I'll move on to a 24 VAC to 12 VDC converter (less than US$10 on eBay) with appropriate series resistor.
I really don't want anything brighter. We're in the country, with homes well spaced, but brighter lights might alarm or annoy the neighbors, especially at night.
The pic is a shot of the pump house with some indicator/trouble LEDs on the outside wall. There are more on another side of the building. They are all flashers of different single colors or alternating colors. Even in daytime, they can be seen 50 m away and much further at night.
The housing is a short, threaded PVC plumbing nipple with a PVC conduit bushing on each end. Between the bushing and nipple on the outside end is a rubber disc with a small slit. The dome of the LED is pushed through the slit up to the flange. These have been in use for 15 years with one failure.
Mike in California
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