Sorry if this is a nube question, but I am working on UV water filtering system for my overland vehicle. There is no built in sensor to determine if the UV bulb is actually on or not. When the bulb is on, it draws 1.5A to 2A from the circuit, but if it is off (or burns out - more important for me) it only draws a tiny amount for the electronics. I would like to monitor the amperage on this circuit so that if it drops even thought the switch is on (indicating a bulb failure), it will sound an alarm, and if it is drawing the amperage then a green LED on control panel will light.
I don't have issues with the programming, but I don't know how to build a a circuit to do such a thing, and then how to hook it to an arduino so I can monitor it.
I hope this question isn't too vague. Basically 1.5 amp drop in 12V circuit and relay gets triggered. LED connected to NC and alarm connected to NO.
One approach would be to put a UV photodiode (or UV LED) in the enclosure, which when the lamp is operating generates a voltage that can be monitored by an analog read operation.
Automotive electrical systems are very hard on electronics, so anything you do to monitor the current directly needs protection from high voltage transients produced by alternator and engine operation, and even possible voltage reversals.
I thought about this but all of the UV photodiodes I can find are quite pricey. It may still be the way I go. The UV is on a completely isolated system from the car and is actually well behaved (for the most part). It's also on its own circuit which helps isolate it as well.
12V/1.5A=8ohms. A small resistor could serve as a current detector. Depending on the device's circuit, a 0.01ohm in series with the ground leg would show 15mv at 1.5A, and 20mV at 2A, and 0.4mV with a 40mA allowance for the tiny about ofr the electronics--all within the ADC range of an Uno.
I've been reading about transient spikes, and I guess I'm underestimating them. Is there a circuit I should be including in my prototypes. I'm rather new at all of this.
Voltage transients are a very serious problem in automotive circuitry, and there are a number of fast switching protective devices you can add to the system, along with other more standard steps, like capacitive/inductive filters.
A simple trick is to use a diode and a 100 Ohm resistor in parallel. Connect one of the diode/resistor pair, the other end goes to your load. Use another 100 Ohm resistor and connect it to the base of a transistor (I use a germanium transistor and the load junction. The emitter goes to ground. What happens is when the load is active the transistor will be turned on and the collector driven low. Remove the load and the collector will rise to that the external voltage to is. Be sure the diode can support the current, the transistor can be a small signal preferably one with a low Be voltage.
This currrent sensor has a 2400 Volt galvanic isolation between the side that is connected in-line in your power wire to the UV bulb and the side that you can connect to your Arduino to provide 5 Volt power to it and read an analog signal voltage (via analogread) that represents the actual current.
I am using the bigger brother of this sensor (acs758) to measure large current consumption in model airplanes.