BBC Focus Magazine

[David Young]

With the aid of a breadboard, wires and crocodile clips, I have set up a basic circuit consisting of a 9-volt battery, a rheostat and a motor. The circuit works, but when I add a supposedly light-sensitive diode to it, the motor runs whether the diode is covered up or not. This is the case both with a conventional photodiode and one which is specifically supposed to work only in visible light.

I thought this was a fairly straightforward circuit, as I remember such arrangements being put together at my secondary school for parents' evenings. However, I'm obviously missing something somewhere.

In order to get a motor running, powered by a PP3 battery, only when the room is in daylight (so if, say, the circuit were in a closed drawer, it would not run), what is the most basic circuit I can construct? Can I still salvage my light-sensitive diodes or should I buy a different component?

[Doonhamer]

Might be best to switch a relay with it. You only need a transistor, a variable resistor (10K), the photo diode and a relay. Put the resistor and diode in series with their junction connected to the base of the transistor to switch it on. You can adjust the resistance to set the trigger light level. Put the relay on the collector side of the transistor.

[The Beige Avenger]

Try turning the photodiodes around... just in case

I've never used them before.

[Fleck]

I made something similar on veroboard with a light dependent resistor switching a BC547c transistor and a LED on when it gets dark, I didn't need to use a relay.
If you could post the circuit diagram I could perhaps point you in the right direction.

[David Young]

The circuit is simply a battery connected to a motor, but with the diode added (and I have tried it with the diode the other way round). I have also tried it with a light-sensitive resistor. The presence or absence of light makes no difference.

Do light-sensitive diodes only work with particularly low currents? In which case, would I need a separate battery for the light-trigger or could this part still be run from the output of the PP3?

[Fleck]

Hmm.. The motor should certainly go off when in darkness because the resistance of the diode will shoot up to millions of ohms in darkness, in fact in bright light it might still be hundreds of ohms which will just make the motor run slow at best.
My guess is that the diode is knackered due to being directly in line with the motor. Get a new one and connect it up via a transistor big enough to power the motor. I'll try and post a simple diagram later.

[Fleck]

As someone suggested the addition of a relay (in place of the motor on the diagram)would make it work better and switch on suddenly, as apart from slowly speeding up. Also dont expect the PP3 to last very long running a motor.

Enjoy!

[David Young]

Thanks for the advice. For the record, I have all three of the photodiodes in a breadboard and I'm testing them with a multimeter (a DT830B).

The photodiodes are labelled BS120.

On the diode setting of the multimeter, the display shows the expected '1' when wired up in reverse, and a display of about 717 for all three (including one that has not been tried in a circuit with the 9V battery yet). When I put my thumb over the sensor window on the diode, the reading drops to 700. However, this does not happen when I cover the sensor with a piece of black plastic, so I'm assuming the change in reading is due to electrical interference from my body.

Is this what I should expect from a photodiode or is there something wrong with the batch? By the way, the resistance setting gives me nothing, but they do react to different levels of light when I set the multimeter to work as a voltmeter.

[Fleck]

I've never used photodiodes just LDRs but reading up on them they are designed to be used in reverse bias and generate a tiny current when exposed to light. So when you were running the motor it would have been foreward biased and unaffected by light.
Use my diagram above, substituting the LDR for your photodiode with the arrow pointing upwards so the diode is reversed biased. While you are prototyping I'd use an LED and 1000 ohm resistor on the collector instead of the motor just to see when the transistor is on. You might have to play about with the resistor values a bit but there's loads of cheap ones on eBay .

[Fleck]

Did you have any success?

[David Young]

Thanks for the help.

I've finally had the time available to rig up the circuit from the photograph above and here are the results:

With an LED it does indeed work, in that the LED shines when the LDR is uncovered and goes out when the LDR is covered.

With a multimeter rigged up to the two crocodile clips that the motor is supposed to be attached to, it registers about 8 volts (the same as the battery) when the LDR is uncovered and zero when covered. This can be altered with the variable resistor.

The motor runs quite fast when 8 volts is applied.

When the motor is connected to the crocodile clips, nothing happens.

When the motor is connected to the crocodile clips via the multimeter, nothing happens but the multimeter registers that 8 volts is passing through it.

When the power supply is doubled to 16 volts, the motor turns slowly and is not in any way affected by whether the LDR is covered or not.

There is one difference between the circuit in the picture and mine: I am using a BC547 rather than a BC337 transistor, as I have been unable to find a BC337. From other forums I gather that this does not make a difference.

Any ideas on what is going on with my circuit?