# I Setting up a circuit to drive a Laser Diode

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1. Apr 6, 2017

### MxwllsPersuasns

Hello fellow physics friends (alliteration: always a strong opener). I am writing today because I find myself struggling to complete something which ostensibly doesn't seem that difficult to the average physics undergrad working in a lab. However, electronics and circuits are definitely a weakness of mine. So I come to you all in the hopes of some resolution.

Basically I need to create a simple circuit that will drive a laser diode. We will have a 6V battery which supplies the current necessary to drive the laser and in the circuit I want a fixed resistor (such that the current never exceeds a pre-set maximum -- for the lasers sake) and then a variable resistor to attenuate the current if need be. In addition I'd like to place an ammeter to measure the current downstream of the resistor(s) and finally I will attach the laser diode leads in order to draw current from the circuit. Now I've got the details of how I'm going to make everything work it's simply the circuit which is an issue for me.

I've attached a picture of a recent attempt I made and I shall explain my (flawed) logic in the hopes someone can elucidate the correct way for me. So I was told the columns of pinholes in the circuitboard are what share the current and the rows all consist of independent pinholes (not sharing the current). Knowing this I basically set it up such that the components were all in one column, thinking the current would flow from the battery to the fixed resistor, then through the variable resistor then the ammeter then into the leads of the Laser (which isn't pictured) but it appears that doesn't quite work.

Unless perhaps the very top two rows (where I placed the leads from the battery) are independent of the other "block" of pinholes underneath it and thus the current from the battery never reached the rest of the circuit? In this case though I notice then that I won't have enough space in one of those "blocks" on pinholes and so I wouldn't have the space to test out my circuit. There must be a way around this, if this is the case, It's just not dawning on me. Any help is GREATLY appreciated!

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2. Apr 6, 2017

### Baluncore

There are two strips along each edge. One marked blue for negative, the other red for positive.
All other connections are only 5 holes long and do not connect across the trench down the middle where you plug in DIP ICs.

If you look under some breadboards you may see the connector strips.
Use a resistance meter to check the continuity.

3. Apr 6, 2017

### MxwllsPersuasns

So if the connections are only 5 holes across and I need say a span of like 8 holes, what can I do? I'll have to look at the circuitboard tomorrow when I'm in the lab again but if only the columns are connected together then how can I keep the circuit connected given that I'm gonna have to use more than one (set of) column(s)? Also that's a great suggestion checking the continuity of the current source using an ohmmeter.

Also do I need to get the polarity correct (i.e., put the lead associated with the positive terminal of the battery to the positive side and likewise for the negative? Or does it not matter?) for the current to flow?

4. Apr 6, 2017

### Baluncore

See attached sketch.

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