Simple transistor as a Switch

Thread Starter


Joined Feb 11, 2013
I am trying to switch ON and OFF a Power supply, using a simple Day Night detection, using a simple LDR sensor.

Even though, the base voltages are correct, The transistors aren't switching properly, Except for the first time, The transistor switches correctly.

Any suggestions.



Joined Aug 21, 2008
We need a better look at the circuit than that. It is not clear and we can't see the load.
Where is the Light Dependent Resistor?
Are those jumpers across the resistors tied to the base?
What is the transistor?

Can't make out the resistor values, but hope to be able to see them on a clearer image.


Joined Aug 10, 2008
Where is the Light Dependent Resistor?
Looks like two in the base voltage divider, one in series with the pull down, the other in parallel with the pull up resistor. After looking at it through a magnifying glass, maybe there not, can't tell with the lableing.

Except for the first time,

It may be the cap across the diode, is keeping positive voltage on the base after charging up. The only discharge path might be through the pull down network, however the resistances in series may not be allowing full discharge. You may need to look at discharging the cap through a different setup, it the cap placement is the problematic.
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