Intermittent operation

We will often encounter the case where we have a small solar cell, but we want to run a motor that needs more power than the cell can provide. To get around this problem, we can store up energy in a capacitor until we have enough to make the motor run, or flash the light. How often it runs will depend on the amount of sunlight there is. On bright days, it might jerk around fairly often. On cloudy days, or late afternoons, it might only move every minute or so.

The circuit we need to make this happen is built around a three-pin integrated circuit, the TC54VC2902EZB, which we'll call the TC54 for short.

This integrated circuit monitors the voltage it sees between pins 2 and 3.

It keeps pin 1 at ground level (zero volts) until the voltage between pins 2 and 3 is above 2.9 volts.

At that point, it raises pin 1 to whatever the voltage is between pin 2 and 3.

To use it, we connect a solar cell in parallel with a capacitor. The solar cell charges the capacitor slowly (because the solar cell isn't big enough to charge it quickly). Pin 2 monitors the rising voltage on the capacitor. When it reaches 2.9 volts, it turns pin 1 on.

Pin 1 has enough current to light a small LED, but not much more. To run a motor, we add a transistor switch, which can handle a lot more current. We use a BC337 NPN transistor.

The pin on the left is the collector. The pin in the middle is the base. The pin on the right is the emitter. If you choose to use a different NPN transistor, remember that the pins may not be in the same order. For example, the 2N4401 has them reversed.

We don't want the TC54 to shut off right away when the voltage drops below 2.9 volts, since that would shut off the motor. We want it to stay on for a bit, so the motor runs long enough to use up the power stored in the capacitor. To make this happen, we add a small capacitor between pins 2 and 3, which stores charge for a bit to keep the TC54 thinking the voltage hasn't dropped yet. We also add a diode between pin 3 and ground. so the TC54 sees a quarter volt on that pin instead of zero volts. These two added parts combine to give the motor some time to run after the TC54 turns it on.

The schematic looks like this:

To build the circuit, we start with a solderless breadboard:

We will use the top row of holes for the positive, and the bottom row for the negative (ground) connections. The first step is to add a wire from the positive rail to where pin 2 of the TC54 will be.

Next we add the TC54, making sure that pin 2 (the center pin) is in the same column as the wire we just added.

Next we add the small 0.47 microFarad capacitor, putting its negative lead into the same column as the TC54's pin 3. The capacitor's positive lead goes in the same column as the TC54's pin 2.

Next we add the diode. It will connect pin 3 of the TC54 to the emitter lead of the transistor we will add in a bit.

We now add a wire from pin 1 of the TC54 to where the base of the transistor will be.

Now we add the transistor. The pins are (from left to right) collector, base, and emitter.

We now add a wire connecting the emitter to ground.

The big 4700 microFarad capacitor is connected between the positive rail and ground. It has a positive lead at the top of the picture, and a negative lead at the bottom.

We can now add the solar cell. The positive lead (red) goes to the positive rail, and the negative lead (black) goes to ground.

The last step is to add the motor. One lead goes to the positive rail, and the other lead goes to the collector of the BC337 transistor.

At this point, the solar cell is charging the 4700 microFarad capacitor. When it gets to 2.9 volts, the motor will spin for a little bit, and then stop. The capacitor will charge up again, and the cycle will repeat. In bright sunlight, the motor may even run continuously, since the voltage will never drop below 2.9 volts. But in the shade, when there would not be enough light to run the motor at all without our clever circuit, the motor will now run for a little bit every few seconds.

The TC54 comes in many different voltages, so you can experiment with higher or lower voltage trip points. This may be useful if you have higher voltage needs, or a lower voltage solar cell.

You can also use a different chip, such as the MCP112.

In the video shown below, you can see a sculpture made using this circuit. The parts have been soldered together "free form", rather than using a solderless breadboard, or even a printed circuit board. They hang together in mid-air, in what we have been calling SteamPunk Electronics form. You can see the solar panel attached to the two support wires, and the large capacitor soldered across those two wires. The TC54, BC337, 1n914, and the small capacitor have their respective pins soldered together so they are connected in the same way as they are on the solderless breadboard.

While meant to be amusing, the sculpture can actually be made to ring only when the sun is in the right position, so it can serve as a functional alarm clock. Just place some shading objects in the right place.

Notice that as the sculpture is turned, the rate at which it rings depends on the amount of light hitting the solar cell. The circuit gets enough light even indoors that it will ding every few seconds.