Need help with photocell wiring

[dwilkin3]

Hello all,

I am an engineering student working on a project for a class. The basis of the project is an optical sorter which seperates white ping pong balls from black ones. I have a photocell which needs to trigger a small 6V dc motor that moves a flipper. I can't figure out how to use the photocell to trigger the motor. The minimal resistance of the PC in bright light is 2.5K Ohms and the max is somewhere around 50K Ohms. My project needs to be very basic and not use any kind of a programmer, just simple circuitry and parts. I thought maybe i could use the photocell to excite a relay that switches the motor, but i don't know if this is foolish. Any help would be a great help. Thanks.

 

[MikeMl]

Hello all,

I am an engineering student working on a project for a class. The basis of the project is an optical sorter which seperates white ping pong balls from black ones. I have a photocell which needs to trigger a small 6V dc motor that moves a flipper. I can't figure out how to use the photocell to trigger the motor. The minimal resistance of the PC in bright light is 2.5K Ohms and the max is somewhere around 50K Ohms. My project needs to be very basic and not use any kind of a programmer, just simple circuitry and parts. I thought maybe i could use the photocell to excite a relay that switches the motor, but i don't know if this is foolish. Any help would be a great help. Thanks.

Here is a simple circuit that does what you need. The circuit operates from 6V. I'm assuming you have a suitable 6V relay.
The circuit has hysteresis, which means the relay will pull in without chattering.

The simulation shows what happens as the resistance of the photocell {PC} varies (x-axis). I'm using a pot to let you select the illumination level that causes the relay to pull in. The five sim runs are at {Pot=10K, 30K, 50K, 70K and 90K}.

 

[dwilkin3]

Thanks so much Mike, i really appreciate it. I just have a couple of questions, i'm a mechanical student so you'll have to bear with me for circuitry stuff. Is the relay represented by the parallel circuit with the diode and the inductor or is it just the inductor part? Also is the positive terminal to the motor represented by the terminating wire called "out"? and finally what is the arrow at the very bottom of the circuit, is that just a ground?

 

[dwilkin3]

also i do not already have a relay and could use a little guidance in choosing one from radioshack. Thanks again.

 

[MikeMl]

Thanks so much Mike, i really appreciate it. I just have a couple of questions, i'm a mechanical student so you'll have to bear with me for circuitry stuff. Is the relay represented by the parallel circuit with the diode and the inductor or is it just the inductor part? Also is the positive terminal to the motor represented by the terminating wire called "out"? and finally what is the arrow at the very bottom of the circuit, is that just a ground?

The relay coil is L1. The diode is something you must add to suppress the inductive kick that would otherwise occur if the diode is not there. You can use a 1N4001 thru 1N4007 for the diode. The relay coil should have a resistance of => 40Ω at 6V.

If you cant find a 6V relay, 12V automotive relays are cheap and plentiful at an auto-recycler. If you use a 12V relay, you will have to power the circuit from 12V, while the motor only gets 6V, so you will need two power supplies. The circuit will work from 12V, but resistor R5 needs to be raised to ~100K.

The "Load" is just a node label. It has no electrical significance other than the voltage at that node can be plotted in the simulator software. The contacts on the relay, and how they are wired to the motor is not shown in the schematic. The Normally Open contact is wired in series with the motor to make it run when the relay is pulled in.

The symbol at the bottom is just a ground symbol. The simulator requires that you designate a "reference node" which all voltages are plotted with respect to...

 

[MOSFET KILLER]

I built a similar circuit using an LM393 comparator and a photocell set up with a resistor as a voltage divider, the circuit turned on a transistor when the voltage went higher than the reference voltage. I used this to make a LASER alarm and it works pretty well.