New to circuits: Capacitor/ Transistor Circuit Help

[SteveNJ]

Hey Guys,

Let me start off by saying that I am a software developer with only the basic knowledge of circuit design. I have started messing around with some projects to get more familiar. What I am trying to do is flash an LED bright for a short period of time by charging a capacitor with a battery and then when a switch is pressed discharging the capacitor quickly. I have designed a basic circuit with two transistors but I cant seem to get it to work correctly. I just need to know if my basic design is sound. I have used a PNP transistor with a current limiting resistor connected to the base so that when I first connect the battery the capacitor, it immediately charges until fully charged. I have used the NPN transistor so that the LED does not have a connection to the negative terminal. Then when the switch is closed what I was hoping would happen was that the current would flow into the PNP transistor base essentially cutting the capacitor from power and flow into the NPN transistor allowing the LED to connect to the negative side of the capacitor allowing it to discharge quickly. Does anyone see anything completely wrong with my circuit logic ? Any help would be appreciated, thanks.

Steve

[pebe]

Hi Steve,

The PNP transistor seems to be serving no useful purpose so I suggest these mods.

1. Remove the PNP transistor and wire the NPN emitter directly to battery-.
2. Insert a 10K resistor in the battery+ feed to the LED and cap.
3. Connect a high value resistor (100k?) between base and emitter of the NPN.
4. Your existing resistor between switch and base needs to be around 5K to 10K (depending on battery voltage) in order to get a base current of about 1mA. Max collector current will be base current x transistor gain.
5. A note of caution: There is nothing to limit LED current so don’t use a very large cap else the LED will not be able to safely handle its discharge energy.

When you connect the battery, current will then flow through the 10K to charge the cap. Because the base and emitter voltages of the NPN are the same (tied with the 100K resistor) and the switch is off there is no base current. So no collector current and LED is off.

When you close the switch, your existing resistor will pass current into the NPN base. That will switch on the NPN and so connect the LED across the cap. There will be a brief flash as the cap discharges through the LED. The LED will then be unlit because there will not be enough current through the 100K feed from the battery to keep it lit.

When you release the switch the cap will charge up again ready to repeat the experiment.

I hope that has helped.

[SteveNJ]

This worked perfect and was a great starting point for me to build a more complex circuit that I was trying to make.
Appreciate the help.

Steve