Darkness activated circuit add on for child's toy

What?

I hooked up a crawling baby doll toy to a breadboard circuit of my own creation, that redirects current to the motor based on the ambient light condition. (Its a baby doll that crawls when the lights go off.) 

Why?

Good question. I came across the idea while scrolling through the comment section of a Michael Reeves youtube video and thought it would be hilarious, while also being a good way to learn some more electronics and (unsuccessfully) refine my soldering skills.

The Video

The Circuit

Hardware

The components used in my circuit were as follows:

 

§  1 x 9V battery

§  1 x 1KW resistor

§  1 x 10KW resistor

§  1 x LDR

§  1 x diode

§  1 x NE555 IC

§  1 x motor (from doll)

I'm sure you can acquire the rest of the components, but if you're looking into starting little projects like this, I highly recommend this starter kit, it has a large variety of components for different projects and some examples to get you started. 

This is the circuit diagram:

 

How does it work?

The magic of the circuit comes from the LDR and the NE555 IC. 

The LDR changes resistance based on the amount of ambient light shining on it, they're made of a semiconductor material, meaning it has less free electrons and hence a higher resistance than a normal wire (in the dark). 

When light shines on the LDR, photons of light give the electrons more energy to move around the circuit freely, hence the resistance reduces. I utilized this property to have the 'dark activated' function of the circuit. 

This might seem counter-intuitive as if there's less resistance then surely the motor will have a higher current and will turn on when it's brighter? That's where the NE 555 IC comes in.

The IC is composed of two comparators, a flip flop, a voltage divider, a discharge transistor, and an output stage. This YouTube video explains how this IC works extremely well and goes in-depth about its 3 different modes, bistable, monostable, and astable, which is where I learned about them.

In a very basic sense, it compares analog voltages at pins 2 and 6 in the circuit to 1/3 and 2/3 the supply voltage respectively, then outputs all (HIGH) or none (LOW) of the supply voltage to pin 3 depending on which one is greater. 

Here, pins 2 and 6 are always at the same voltage, which will change depending on the resistance of the LDR, if the resistance of the LDR is high enough then the voltage here will be low enough compared to the supply voltage and we can get a HIGH output (baby turns on).

The Doll

After some research, it seemed the Tak May crawling and talking baby doll was the one for the job because it already has the crawling capability. I didn't want to have to do any animatronics to get the doll crawling, just to harvest the crawling capability in my own way. The talking feature was just the cherry on top.

It seemed to be the best on the market but despite how funny it is, I didn't want to spend £50 on the doll so I somewhat suspiciously (according to my parents) bought one off of eBay for around £14.

Once the doll arrived I took it apart to find out where the motor attaches to the circuit board. I simply cut the wires connected to the motor (red and green wires pictured below) and soldered them to pinned wires, so they would easily connect to my breadboard. 

Obstacles & How they were overcome

LDR's are by definition have high resistance when the ambient light is low, we want the opposite effect which required a slightly more complicated circuit. Luckily, it is a common DIY project to use this kind of circuit to automate garden lights turning on so there was plenty of information on how to set it up.

Unluckily, none of these DIY projects that I could find powered a motor, just an LED or a relay. When I tried hooking up one of these circuits to the doll's motor, it would work then gradually slow down until it stopped. After some research, it turned out this was caused by feedback coming from the motor. To counter this I placed a diode between the motor and the IC meaning current could only flow in the direction I wanted it to. 

No issues after that, a successfully creepy doll, confirmed by my parents, a few Reddit users, and my mates - luckily none of whom have disowned me for it.