Astable multivibrators

[ExpAnonColin]

In general, how are the ramps on a 2n3904 astable?  Driving an LED it seems less than perfect.  Would putting a hex inverting schmidt trigger help, or is it the LED?

Also, in one of my books it says that an inperfect sawtooth is produced in between the negative end of your capacitor and the base of either transistor...  I tried it, but no cigar.  Anyone?

-Colin

[gez]

In general, how are the ramps on a 2n3904 astable?  Driving an LED it seems less than perfect.  Would putting a hex inverting schmidt trigger help, or is it the LED?

Also, in one of my books it says that an inperfect sawtooth is produced in between the negative end of your capacitor and the base of either transistor...  I tried it, but no cigar.  Anyone?

-Colin

Not too sure if I've understood correctly.  Are you using the output (square wave) to drive the LED directly?

Whatever you're doing, it would be advisable to control the LED with a FET or trannie driven by the astable and not connect it directly.  Another possibility would be to use a inverter or a Schmidtt to buffer the astable's output, but then you might as well use one of the other Schmidtt triggers in a chip for the oscillator (or wire up a couple of inverters to do the same).

In many oscillators you can tap the junction of the cap/resistor which sets the rate to get a not linear triangle - convex curve as it ramps up, concave as the cap discharges.  All you're doing in effect is integrating a square/rectangle wave.  The amplitude varies with frequency so it's a pain in things like tremolo/auto wahs as you need to readjust the depth whenever you change the speed.  However, they can be useful! (I've just use a couple of passive integrators to create a true sawtooth in a octave up effect - sings nicely!)

[ExpAnonColin]

Whatever you're doing, it would be advisable to control the LED with a FET or trannie driven by the astable and not connect it directly.  Another possibility would be to use a inverter or a Schmidtt to buffer the astable's output, but then you might as well use one of the other Schmidtt triggers in a chip for the oscillator (or wire up a couple of inverters to do the same).

In many oscillators you can tap the junction of the cap/resistor which sets the rate to get a not linear triangle - convex curve as it ramps up, concave as the cap discharges.  All you're doing in effect is integrating a square/rectangle wave.  The amplitude varies with frequency so it's a pain in things like tremolo/auto wahs as you need to readjust the depth whenever you change the speed.  However, they can be useful! (I've just use a couple of passive integrators to create a true sawtooth in a octave up effect - sings nicely!)

Yes, the LED is being driven directly.  I'll try to move it away, but it's been working fine so far... what are the disadvantages?

Thanks for clearing the sawtooth stuff up!

-Colin

[ExpAnonColin]

Whatever you're doing, it would be advisable to control the LED with a FET or trannie driven by the astable and not connect it directly.  Another possibility would be to use a inverter or a Schmidtt to buffer the astable's output, but then you might as well use one of the other Schmidtt triggers in a chip for the oscillator (or wire up a couple of inverters to do the same).

In many oscillators you can tap the junction of the cap/resistor which sets the rate to get a not linear triangle - convex curve as it ramps up, concave as the cap discharges.  All you're doing in effect is integrating a square/rectangle wave.  The amplitude varies with frequency so it's a pain in things like tremolo/auto wahs as you need to readjust the depth whenever you change the speed.  However, they can be useful! (I've just use a couple of passive integrators to create a true sawtooth in a octave up effect - sings nicely!)

Yes, the LED is being driven directly.  I'll try to move it away, but it's been working fine so far... what are the disadvantages?

Thanks for clearing the sawtooth stuff up!

-Colin

[gez]

Yes, the LED is being driven directly.  I'll try to move it away, but it's been working fine so far... what are the disadvantages?

It depends on the oscillator, with some it's not a big deal with others it screws with the timing.  A lot of the time it can be factored in and compensated for if it does affect the timing.  As far as the astable goes, if it works then fine...