[1/8/21 – See bottom of this post for update on question]
My son and I have been enjoying Elenco’s Snap Circuits (my review) for quite some time – we’ve completed over 50 projects! Recently we did a project that uses a push button switch to turn on/off a motor which has a paper spinner on top that rotates. Below is our build of this fairly simple circuit with the spinner on:
Here is the same circuit with the spinner removed so one can see all the details of the circuit:
This all works great! Now much precocious four year old suggests that we should be able to replace the push button with a photo resistor to control whether the circuit is open or closed. Makes sense to me, so we shuffle the parts like so:
Now we try shining a light on the photo resistor – nothing happens. I try flipping the photo resistor, still nothing. I move back to the switch – it works like a charm. What am I missing?
Update – 1/8/21
Some friendly folks on Quora have pointed out that a switch essentially eliminates resistance when closed but that a photo resistor, even when activated, still creates resistance. This is helpful but we still have a bit of confusion. Elenco offers a book of projects, below I’ve included an image of Projects 19-21:
In Project 19 we build a circuit that uses a traditional switch. In Project 20 we are instructed to swap out the switch (S1) with the photo resistor. This project works. What is it about this project that allows it to work while our modified project above does not?
The motor needs dozens or hundreds of milliamperes to run. The 3volt battery voltage cannot push that much current through the thousands of ohms (killo-ohms) of the LDR.
Any switch should have less than one or two ohms when it ‘makes’ the circuit, that allows enough current for the motor from the batteries.
A switch that is ‘open’ has millions or billions of ohms and will only allow a micro-ampere (maybe only nano or pico amps) to flow.
Thanks William!