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I'm working on a little project(an infrared jammer) and I'm having a few issues.

Why (and how) exactly do circuits like the one below use electrolytic capacitors? Wouldn't ceramic capacitors be suitable for this?

Also in the NPN configuration, why are the LEDs placed with the collector and not the emitter?

If I were to make the frequency variable by using a potentiometer in place of the 39K resistor would the duty cycle change as well as the frequency?

Astable Multivibrator

I'd appreciate any help! Thanks.

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1 Answer

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Any type of cap is fine for this circuit - it shows electrolytics in the schematic due to them being cheaply available in large values (e.g. >10uF) Previously you could not get ceramics of this size, and they are more expensive for 10uF.

It's more usual to place the LED on the collector, for NPN and PNP as you can see with both circuits. Placing on the emitter side is fine, but you have to take into account the raised base voltage to turn the transistor on (Vled + Vbe = Vled + ~0.7V) and also raised saturation voltage (e.g. when transistor is fully on, which will be around Vled + ~0.2V)

The duty cycle will change if you only use a pot in place of one resistor - to keep the duty cycle the same you would need a dual pot to change both 39K resistors at once to keep the values the same.

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Thank you for replying. I plan to use 10nF caps with high resistor values at the base. That's ok right? For my application it might make it a bit more confusing to calculate the frequencies but would using a single pot still be acceptable? (to get a range from around 30kHz to 45kHz or so) –  Ammar Nov 3 '12 at 17:10
@Ammar - It is really for you to determine if the duty cycle change due to using a single pot would be suitable for your application. Sometimes when the expected result is experimental, as is likely the case for an IR jammer, you have to just go ahead and build up the circuit and see if it works to your satisfaction. –  Michael Karas Nov 3 '12 at 18:33
@Ammar - Yes, you can use 10nF caps if you want to if the target frequency is higher. I would keep the resistor values below 100kohm, which you will be doing if you want a frequency of around 30-45kHz. As Michael says you'll have to decide whether changing the duty cycle is acceptable (I would imagine it's probably okay for a basic IR jammer, the main thing is just to create some "noise" at the carrier frequency) –  Oli Glaser Nov 3 '12 at 18:38
@OliGlaser - Well I got it working. I might decide to use a dual gang pot or something. I'm going to mark this as solved but I have one little question. These circuits are using common emitter configurations. You mainly use these for voltage gain or inverted output. The voltage gain is -VC/VB (IC*RC / IB *RB). So basically the voltage gain is really small and we're using the inverted output for switching right? Thank you for all the help! –  Ammar Nov 4 '12 at 11:11
Since the transistor is set up as a switch, we can look at it a bit differently. The dynamic voltage gain is large as would be expected with this configuration. If you look around the point of switching, between the base voltage changing from ~0.5V to ~0.75V, the collector voltage will drop from +V to near ground and the current will go from 0 to ~+V/Rc, so the dynamic gain will rise. At other times it will drop to ~0 as things are static. It's not so helpful to look at it this way though. Google for "the transistor as a switch", you should find some helpful stuff to read. –  Oli Glaser Nov 4 '12 at 11:37
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