transistor timers

[daviddoria]

i built this
http://bandtank.com/~calculusap/2 transistor flasher.wmf

and it worked

so to make sure i knew what was going on, i tried to build this
http://bandtank.com/~calculusap/1 transistor flasher.wmf

and the LED just stayed on.

can someone look at the 1 transistor diagram and see what i did wrong?

thanks
david

 

[hantto]

To make a flasher you allways need atleast two transistors, what you have made there is just a led that is controlled via a transistor.

 

[daviddoria]

but what if i only want 1 flashing light? do i just connect a jumper from Vs to the collector of the 2nd transistor?

i thought the whole point was the transistor stopped the light from being on until the capacitor charged... so why doesn't it work with only 1??

 

[kinjalgp]

Hi,
The circuit won't work with one transistor because for a circuit to work as an oscillator there should be a positive feedback present which is only possible using two transistors which cause alternate charging and discharging of the two capacitors and thereby turning the transistors alternately.

If you want only one LED to blink, you can use the same circuit but just replace the other LED by a small resistor of 470 Ohm or so. Thus the circuit will have only one LED.

 

[Gene]

The secret is the charging and discharging of C1. In the second circuit, C1 never changes state. The problem is at C1.