# Using a Capacitor To prolong a pulse

I'm new to electronics and am building a very simple circuit involving a peizo speaker, amplifier IC and LED.

Right now I run the peizo into the amp IC and then take the output to the led. With this when I bang the peizo on the table the LED lights up for a briefly. I'd now like to modify this so instead of the LED immediately going off I'd like it to stay on for a bit longer.

My thinking is to do this I need to use a capacitor and have the peizo trigger the capacitor to dump power into the LED. I'm stuck however trying to think through the circuit. How can I charge up the capacitor and then once fully charged stop the flow of power to the capacitor and then dump it out another path when the peizo triggers?

My thinking is I could use an NPN and have the peizo trigger the gate and then have the capacitor keep the gate open while it also dumps into the LED but I'm still not sure how to initally charge the capacitor.

• Could you use the circuit editor and draw a circuit? – Voltage Spike Feb 16 '16 at 17:43
• Go do a google search on RC circuits. Also if you want a cool way to generate a PWM there are these chips linear.com/products/TimerBlox – Voltage Spike Feb 16 '16 at 17:43
• Try looking up "monostable". – Andy aka Feb 16 '16 at 17:43

You've left us a lot of guesswork due to lack of schematic.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Rectified amplifier output and smoothing capacitor.

How it works:

• D1 rectifies the amplifier output.
• C1 (1000 uF in the schematic - use whatever big ones you've got) stores charge between pulses from the amplifier.
• R1 and D2 you should have already.

How well it works depends on the amplifier and the size of the capacitor.

If there's an output capacitor on your amplifier you'll need the circuit shown in Figure 2.

simulate this circuit

Figure 2. Modified circuit for capacitor coupled output.

• D3 is required in this case to "recharge" C2 on negative half-cycles
• Your first schematic makes sense, but when I "bang" the piezo against the desk it create a very fast spike of power. Will the shortness of this spike provide enough time to charge the C1 capacitor? – Mikeb Feb 16 '16 at 21:26
• How could I possibly know? You haven't given a circuit as requested in the comments and you haven't stated what kind of amplifier it is. When you bang the piezo it probably gives a short-duration high-frequency oscillation so there will be multiple cycles per bang. At worst the amplifier will "current-limit" at whatever its maximum output current capability is. – Transistor Feb 16 '16 at 21:32
• added schematic – Mikeb Feb 17 '16 at 1:01