I'm using Arduino and relay shield to control five 24 VDC Solenoid Valves.
Now I want to place an indicator light (LED) for each valve, so whenever the valve is open, the light comes on.
Do I need 24VDC LED lights in order to do so?
Electronic – arduino – How to place an indicator light for 24 VDC Solenoid Valve
arduinoindicatorledsolenoid
Related Solutions
This isn't too hard to implement. I can see the box and controller getting out of sync, but if the controller hits the zone twice and it doesn't matter what order the two sub-zones come on in, then that seems fine.
You are right in that you need some kind of memory. Since the unit will have no power to it between uses, that memory needs to be non-volatile. A microcontroller with built in EEPROM would do fine. EEPROMs are only good for a finite number of writes, but that's 100s of 1000s at least so no issue there.
When the power to the switch box turns on, all it really does is run the micro. The micro then turns on one of N relays to route the power to one of the sub-zones. It also writes the new state to its EEPROM so that it will power the next sub-zone in sequence next time.
A tiny micro running at slow clock speed can easily handle this. The 5V current will be small so a linear regulator will do well enough and be simple. Get relays that can run directly from the full wave rectified AC so there are no power conversion issues. 24V AC after full wave bridge with filter cap should be around 30-32 Volts. "24V" DC relays would work but get a little warm. Genuine 30V relays may be harder to find, so you could get 24V relays and put a resistor in series with the coil. A reverse catch diode accross the coil and a NPN transistor with base resistor to the micro is all you need per sub-zone output.
Another thing to consider is that the micro needs to see one power up each time the main controller turns on the zone. This should be as simple as putting a little low pass filtering on the micro's reset input so that it doesn't start running until a 100 ms or so after power is applied. By that time glitches and switching transients should be over.
The main controller also needs to leave some off time between powering this zone so that it toggles to the next sub-zone. It will take some time for the voltage to drop before the micro loses power or is shut down by the reset circuit. It could be a second or two depending on what values are chosen.
The more I think about it, the more I'm realizing the trickiest part of this is the reset circuit. You want to make sure the micro runs cleanly once per power up, and that it goes into reset cleanly once on power down and not too long after power down. This is all quite doable, but something that needs to be considered.
- Schematic needs part numbers or a BOM attached. eg what type of cap?
- Card interface needs protection for ESD sensitive devices on interface pins such as MOSFET gates. or a warning label.
- Relay contacts may need ferrite filters to reduce ingress of signals from sprinkler wires (antennae) and internal electronics. CM ferrite choke recommended.
- Also contacts can create peak AC transients to be radiated from interface cables to nearby electronics connected to this card.
- Since these are not zero crossing switched solenoids, it is hard to say if there will be any EMC surge ingress issues at a system level but filtering or shielding may be necessary and use twisted pair wires. Otherwise, contacts may be heard on AM radio which you can use off channel as a quick test. concur with other suggestions.
- An LED indicator & resistor on board and each driver may be useful. 3mA or so should be adequate with any low cost AlGaAs indicator rated for 20mA.
- Schematic needs legible notes for interfacing requirements, test points
- Galavanized nails are highly unreliable for reliable plant moisture sensors but for crude watering, it may be acceptable. Capacitance mode is more reliable than resistive mode for linear thresholds due to drift in soil shrinkage around sensor, galvanic soil offset, AC hum, RF noise etc.. I hope you have done some research.
It is hard to read, but possible.
Best Answer
There is no such thing as a "24 V LED". However, you can use a resistor in series with a LED so that the combination can be safely lit from 24 V. Let's say you want 5 mA thru the LED, and that the LED drops 2.1 V (typical of normal green LEDs). From Ohm's law, (24V - 2.1V) / 5 mA = 4.4 kΩ, which is roughly a suitable resistance.
Keep the power dissipation in mind. The resistor will drop about 22 V, which times 5 mA is 110 mW. That's getting close to the limit of a "1/8 W" resistor, so either make sure a 1/8 W resistor is properly mounted at the right temperature or use a 1/2 W resistor. Note that the power dissipation in the resitor is proportional to the LED current, which is one consideration for keeping the current low.