3V Input to 5V/TTL Converter

This circuit converts 3V input to be connect to 5V or TTL device. This is a very simple common-base configuration circuit and uses 4 components only. R1 and R3 are used to pull-up voltage to 3V and 5V respectively if no 3V input is present. While R2 is used to limit the base current of Q1 when its start to conduct when the input becomes low.

Telephone Ring Indicator Using LED Lamp

This is the circuit for Telephone Ring Indicator Using LED Lamp. The telephone ring is typically 90VAC RMS so is more than enough to light a single LED. C1 is used for DC blocking thus no OFF-HOOK condition at standby state. R1 is to dissipate excess voltage, while D1 and C2 rectifies it to positive DC voltage. D2 regulates the voltage to 9VDC with R2 again limits the current of the LED during ringing.

220V Line Powered LED Lamp

To connect a single LED lamp to the 220V AC line, the minimum need are a series a high voltage capacitor and a resistor. A diode also in parallel with the LED in reverse polarity connection.

The diode prevents reverse voltage breakdown of the LED. The resistor is needed to prevent in-rush (surge) current to the capacitor if from OFF (discharged). An additional resistor in parallel to the capacitor to bleed the charged voltage when the circuit is OFF to prevent electric shock.

Computing for values:
I(led) = 0.02
Xc = V/I(led) = (220-3)/.02 = 10850
Cmin = 1/(2*pi*Xc) = 1/(2 *pi * 60 * 10850) = 0.24uF = 0.22uF nearest value