LED as resistor when adding DC input?

xorbelbeeb · February 04, 2010, 08:08:12 PM

Hi!
I bought a mini-theremin kit and am pimping it out a bit (new metal case - better antenna - 1/4" out) and I'd also like to add a DC out. The thing normally runs on 4 aa batteries (so a total of 6V). I have an adapter rated to 7.5V and was thinking I could add in an LED to draw the voltage down to 6 before it reaches the circuit. Would that work? would I still need a resistor for the LED? Help

Gordonjcp · February 09, 2010, 12:11:59 AM

Can you post the circuit for the kit? It may already have a voltage regulator, in which case you don't need to worry.

I would suspect that an LED would drop a little too much voltage (Vf is typically around 2V) and would only pass a few milliamps - if your theremin has a built-in speaker it would draw too much current.

xorbelbeeb · February 13, 2010, 08:37:22 PM

I scoured the web and finally found this schematic for it.

Looks like power is routed directly to the IC chip. Like I said, the adapter I have is 7.5V and the circuit calls for 6V. Can I just insert enough resistance to allow only 6V through, or do I need some sort of regulator? $:-\$

Thanks for any help!

xorbelbeeb · February 13, 2010, 08:42:13 PM

hmmm... I had an image there when I previewed. Anyway, here is an attachment with the schematic!

Gordonjcp · February 13, 2010, 11:48:32 PM

It doesn't say what the amplifier chip is, but from the pinout it looks like an LM386. That will handle 7.5V with no problems. You may need to fiddle with the oscillator tuning a little.

xorbelbeeb · February 14, 2010, 09:02:36 PM

Right you are - it's a clone anyway (JRC 386). I'm still a bit new to this so can you be more specific with oscillator tuning?

Thanks for pointing me in the right direction!

Circuitbenders · February 14, 2010, 09:28:25 PM

Quote from: xorbelbeeb on February 04, 2010, 08:08:12 PM
I bought a mini-theremin kit and am pimping it out a bit (new metal case - better antenna - 1/4" out)

Is it a good idea to have a metal case for a theremin?

Mine had to be callibrated away from any metal objects, even my metal workbench that i was putting it together on was altering the pitch response massively, although if the case is always in the same position relative to the antenna maybe it won't matter.

xorbelbeeb · February 14, 2010, 11:03:54 PM

Heh heh

I've had to rethink the metal case (mostly because it's damn difficult for me to cut clean holes in it) - so it'll most likely be plastic or wood. I had thought to mount all the internal goodies and the antenna in such a way as to not come in contact with the metal, but in the end it's probably just a lot safer to go with a non-conductive case.

jamiewoody · February 16, 2010, 08:51:17 PM

i am looking forward to learning to rectify voltage.

if i had my way, everything i do would run on a 9 volt battery, simply because there are no annoying springs, and experimenting later is easier with a battery clip. lol!

Gordonjcp · February 18, 2010, 03:35:38 PM

Possibly you mean "regulate". Anyway, you can get battery holders that take four AA cells (we used to call them HP7s; "AA" is a horrible Americanism. And I remember when this was all fields, you know...) including some with a PP3-style terminals (why did we keep PP3 and start calling HP7s "AA"? And what ever happened to PP6es and PP9s? I need a PP9 for an old Roberts radio from the 1960s).

xorbelbeeb · February 18, 2010, 05:31:20 PM

Here's the situation currently (no pun intended). *insert Alf-style "Ha!"* Ahem.... Gakken mini-Theremin is mounted, new antenna scrapped from a broken radio is attached, 1/4" output is installed, and the last bit is a DC input jack for an adapter that I'd like to use. I clipped the battery compartment leads and tried to attach them to an input jack I cannibalized from a broken cd player. It's a little plastic bugger with 3 pins with which it connected to the cd player's circuit board. Should this work? Do I need to have all 3 pins connected to something (I assume the 3rd pin is ground?)