555 Timer circuits

[DaveLdn]

Hi,
I'm experimenting with 555 timer circuits and have made two test circuits. The first works just fine but the second won't. I'm sure the reason is solution is simple but the problem is driving me mad. Please could you tell me why circuit 2 won't work (hopefully attachments are included!)

Obviously I haven't included the full circuit diagram but the idea is simple. In the first picture, the 555 timer circuit gives an input to the transistor causing it to pulse on and off.

The second circuit attempts to do the same thing using a separate power supply for the latter half of the circuit.
All help greatly appreciated.

Ta
Dave

 

[Resqueline]

There's no common ground in the second picture so the current from the 555 output trying to go into the base and through to the emitter has no way to return to the 555.
I'd also recommend using a resistor in series with the base to define a base current. As it's now the base gets the full output short-circuit current from the 555. This wastes battery life/ power.

 

[DaveLdn]

Thanks for the quick reply. I'll go back to my 555 theory in order to fully understand how it works. Thanks very much.

 

[andreapg]

I have never seen this circuit. What does it do?

 

[(*steve*)]

I have never seen this circuit. What does it do?

I presume the original poster only drew the important parts of the circuit. The timing resistor(s) and capacitor were simply not shown.

The entire issue seems to be coupling the output of the 555 to a separately powered load via a transistor.

 

[DaveLdn]

Hi there,
Sorry, I haven't been on here for a while. The circuit was to test a 555 timer input to an independent circuit without sharing a common ground connection. I was using it as a high voltage source for a voltage multiplier.
Thanks for your help guys.

 

[(*steve*)]

Hi there,
Sorry, I haven't been on here for a while. The circuit was to test a 555 timer input to an independent circuit without sharing a common ground connection. I was using it as a high voltage source for a voltage multiplier.
Thanks for your help guys.

If you don't want a common ground, you can use an optocoupler. One of these will allow many hundreds (in some cases a couple of thousand) volts difference between the two circuits.

 

[Shyamal796]

Give your full circuit. Use base resistor and connect the common ground to complete your ckt.