[How-To] Monitor a device using a status light


Staff member
How to monitor the status of a device using its status light
by electron

About a year ago, I decided I wanted to monitor the status of my dryer and washer. Some people have suggested to just use a remote to indicate when the cycle starts, and then just play an announcement after a previously timed amount of minutes. Since this solution doesn't work well when using different program settings, I wanted something which could actually detect when the machines are running.

The dryer was actually pretty easy. I simply installed a magnet on the drum, then the other part of the magnetic reed switch was mounted on one of the sides inside the dryer, so when the drum makes 1 revolution, it will pass right by the switch, triggering the contact. I hooked this switch up to my SECU-16, and use this method to successfully announce when the dryer started, and when the cycle has completed. This setup will also work with other I/O interfaces such as the gameport, you just need to add some logic to detect when the drum stopped spinning.

Next, my washer. Initially I was going to open my washer up, and see if I could somehow mount a magnet behind the 'program knob' and have it trigger when the knob is in the OFF position. Unfortunately, my washer machine died around the same time, so I never got a chance to try this. Since my new washer is still under warranty, I decided against opening up the new washer (and to avoid a huge WAF drop) and look for other methods.

Then I realized that my new washer has a nice status LED (or bulb, not sure what it is), which is on whenever the washer is in use. If I could only open that washer and tie in to that LED directly ... Of course, this wasn't going to happen, so someone suggested to try to use a photoresistor. I ended up buying the Cds Photoresistor Multi-Pack from RadioShack, for around $3 (part # is 276-1657).

The multi-pack comes with 5 sensors, all different sizes, so I just picked the biggest one, which happened to be around the same size as the status light, and attached it to the washer using black electrical tape to block out any ambient light. Since I have an Ocelot with a SECU-16 I/O module, I decided to directly connect this photoresistor to a SECU-16 input, configured as a digital input.

I turned the washer on, and the Ocelot successfully reported the status change. After a few minutes I noticed that it would hesitate and report OFF, eventho the machine is still running, and then switch back to ON a few seconds later. I figured maybe the status light isn't as reliable and flickers once in a while, so I decided to do some more testing. I got my voltmeter out and measured the voltage in both the ON and OFF condition. When the washer wasn't running, I measured 5 VDC, when the washer was running, I measured 3.10 VDC. I posted this information in the chat room, and was then told to install a 4.7k Ohm resistor in parallel with the photoresistor. I installed the resistor, and this time the measurements were 4.12 VDC and 2.12 VDC. This makes the difference a bit larger, making it easier for the digital input to see when the the washer is ON/OFF.

I am happy to report that after almost 2 months of using this setup, that it has been 100% accurate, and only cost me 3 bucks in materials. I assume that this would work with other input sensors as well (such as the PowerFlash, GamePort, etc.), but I haven't tested this.

Here's the schematic:

If you have other appliances which have a status light, then you might as well start monitoring those considering how cheap these photoresistors are.

Many thanks go to the Cocooners who helped me figure this out and improve my WAF significantly.
I can confirm that this works with the powerflash unit (like there was any doubt) :)

Thanks for the idea and how-to!
Now I just need to figure out what I'm going to monitor :D
i wanted to do this but didn't have an outlet nearby for a powerflash nor did i have a nearby wire run to my controller. so i snipped the cds cell from a hawkeye motion sensor, extended it with some wire & hid the hawkeye. it works nicely :blink:
there is a slight delay i think, less than a second. doesn't matter in this situation where i'm monitoring a light that stays on for a period of time (not switching on & off continuously).
Wouldn't a DS10A work for this application without having to mod a motion sensor?

ver0776 said:
Wouldn't a DS10A work for this application without having to mod a motion sensor?

the ds10a reports contact closure. i don't have a relay on my dishwasher to trigger a ds10a nor did i want to build a circuit to trigger a relay when the dishwasher's "clean" light is on.
there is a slight delay i think, less than a second
Cool. I have another hack that I will be doing soon and was interested in any delays on the CdS circuit.
TonyNo said:
there is a slight delay i think, less than a second
Cool. I have another hack that I will be doing soon and was interested in any delays on the CdS circuit.
i double checked last night & actually it's about 2 seconds from the time the LED i'm monitoring turns on & the LED on the hawkeye blinks (meaning it saw a change in light level)
Hi Guys

I designed a couple of "Probes" to monitor status of appliances around the house....

I use Homevision as my HA Controller and was using Flags to keep track of my tv,s dvd players etc , but if devices turned on / off manually then incorrect status was detected.....;-(( ...Hence the led probe + video probe.....I ended up designing a controller which connects to room Monitor boxes via cat5 and connects to Homevision via i/o ports but also connects to my pc based HA software xPL via a serial interface....

There was a review done of it and it was reprinted on Home Toys online mag.....

Check out the links if interested in a read




I tried this but it doesn't seem to be sensitive enough to light for my application. I am trying to monitor the mute LED light on an A/V receiver. The LED is not very bright when on. Any circuit ideas on how to make the photocell more sensitive to light?

The other option (I would like to avoid if possible) I have is to open up the A/V and monitor the LED directly. What would I need and how would I do this?


I'm now using some relays and power supply connected in the back of my receivers and cableboxes to my SECU16 input to know theirs power status. That was very simple to build.

But your led detectors tip just gave me an idea... Could i use a photocell over the outical output red light? I don'T know if it possible to get an empty TOSLINK connector and put a light detector in it. It would make a clean looking probe. Nothing clued in the front of the TV. And I don't use the optical out.

I know some of you are using CR Magnets to know if your TV are on or off. I would prefer not to split the power cord. It's a new HD LCD.