# Guidance on a simple electronics project **URL:** \ **Category:** In My Humble Opinion\ **Created:** [February 24, 2011, 5:19pm UTC](https://boards.straightdope.com/t/guidance-on-a-simple-electronics-project/572468 "2011-02-24T17:19:13Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![The\_Evil\_Prince\_Zorte](https://avatars.discourse-cdn.com/v4/letter/t/e95f7d/32.png) [@The\_Evil\_Prince\_Zorte](https://boards.straightdope.com/u/The_Evil_Prince_Zorte)\ **Post date:** [February 24, 2011, 5:19pm UTC](https://boards.straightdope.com/t/guidance-on-a-simple-electronics-project/572468/1 "2011-02-24T17:19:13Z")
I have a tankless water heater, which needs a minimum flow to be activated. If you run the cold water too high or flush a toilet, it is easy drop the hot water pressure enough to get below that minumum flow rate. You won’t find out until the cold water arrives a minute later, and then you have to wait another minute for the hot water to get back through the pipes from the tankless. The controller for the heater is in a closet, and has an LED that lights up whenever it is active. I would like to stick a photocell over the LED, and set up a device that will turn on an LED in the bathroom, and one in the kitchen whenever the tankless is running, and switch off when it is not. Then I can know instantly when I need to adjust the faucets or shower to keep the hot water coming. A wire from the closet to the kitchen would run about 20ft, and a wire to the bathroom would be around 60ft. I don’t really know anything about electronics, so I am looking for some advice on what kind of devices and design I would need. I’ve been wanting to learn, and thought this might be a good beginner’s project. I would like to start with a basic design or approach, and then learn what I need to understand and implement it. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [February 24, 2011, 5:58pm UTC](https://boards.straightdope.com/t/guidance-on-a-simple-electronics-project/572468/2 "2011-02-24T17:58:07Z")
You need a light pipe between the LED and detector. What color is the LED? Reason I’m asking… the detector must be sensitive to the wavelength produced by the LED. A photodiode or phototransistor might work, though (again) you have to make sure it is sensitive to the wavelength produced by the LED. A photocell would probably also work, though I would only use one if you can’t find a photodiode or phototransistor that would work. Once you detect the light from the LED, the rest of the circuit is pretty simple, and will include a few transistors and resistors. ---
**Author:** ![Heyoka13](https://avatars.discourse-cdn.com/v4/letter/h/b9e5f3/32.png) [@Heyoka13](https://boards.straightdope.com/u/Heyoka13)\ **Post date:** [February 24, 2011, 6:02pm UTC](https://boards.straightdope.com/t/guidance-on-a-simple-electronics-project/572468/3 "2011-02-24T18:02:11Z")
Might be easier to run a piece of fiber optic toslink cable. Get the cheapest 20 footer you can find on ebay. Viewing angle and brightness might be a challenge though . . . ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [February 24, 2011, 6:02pm UTC](https://boards.straightdope.com/t/guidance-on-a-simple-electronics-project/572468/4 "2011-02-24T18:02:13Z")
This is a pretty simple design. I would use an OpAmp as a comparator. [This](http://2.bp.blogspot.com/_vfmOyxDCru8/SKC3im7dXgI/AAAAAAAADMo/PSaisM4TiSA/s320-R/Basic_CdS_PhotoCell_Schematic.GIF) circuit is pretty much what I would use. You can run additional LEDs off of the output - just use a dropping resistor for each one. remember that you will need to provide power for the OpAmp - batteries will work to start, but they probably won’t last very long. Try an old cell phone charger as a power supply.