Finding the right resistor for an LED

 This is a problem I've never really got a proper handle on until today, helping one of the students at DJCAD: working out the appropriate resistor for an LED.


Having looked up the datasheet for the LED in question, applied 3 volts and promptly blown it up, I had a look for some more info and found this formula here:


The resistor value, R is given by:

R = (VS - VL) / I

V_S = supply voltage
V_L = LED voltage (usually 2V, but 4V for blue and white LEDs)
I = LED current (e.g. 10mA = 0.01A, or 20mA = 0.02A)

Our supply for the radios is 3V, so I made an assumption and looked up a similar LED on Rapid, where we get our components. The spec showed it had a rated voltage of approx. 2V and current of 30mA (0.03A). Hence we need a resistor of 33.3 Ohms. We didn't have them so tried a 27 Ohm, which appears to work like a dream, although we held it arms length for a while to make sure it didn't blow up again!

Radio tinkering and the ideal pot

I am tutoring an. Industrial Design intro module at DJCAD, the theme of which is basically radio hacking. The unit every student has been given to hack is a Sony ICF-S22, I think the cheapest Sony radio, and one chosen as it appears to be more resilient for hacking than the Lloytron I was briefly using on Niftyradio before it got nicked and I lost interest somewhat!

Mike Shorter astutely pointed out after the studio session on Tuesday that as students might be changing out the volume control, we need to know exactly what spec the pot is in order to work with it. Measuring resistance in circuit is a bitch, and not very useful as you will only ever get the total resistance of a component and all its parallel branches, rather than the individual component. So I carefully removed the combined switch/volume control for measuring. Here it is:

The outer two pins are for the on/off switch, while the central three pins are for the potentiometer. Just after spending some time working the component out of the board I then realised that '50k' is engraved on the white top surface! This can be revealed better by rubbing pencil lead onto the surface. Measuring with the multimeter confirmed that the pot is roughly 50k Ohms, however what the engraving doesn't tell you, but using the multimeter does, is that this is a logarithmic pot, not linear. Jon warned me of this, but I'd never really thought about it - the theory is explained a little here - interesting stuff.

So for you second years: The ideal pot to get is a 50k logarithmic pot. I think the standard pot on order for the electronics workshop is linear 10k, which will also work to a certain extent, but probably won't turn the volume right down, and will sound irregular to the ear as  you alter the volume.