What do I need to make this circuit work?

Question

I’m trying to light two 2.2V 20mA LEDs using a 9 volt battery, a PIR motion sensor and a transistor.

Wired as shown with 9 volts applied at the collector and the 3.3 volt signal from the PIR applied at the base of the transistor, the output is only about 2.6 volts which will only light one LED.

My idiot brain thought the PIR would output a signal when motion is present, then the transistor would act as a switch and I would get approximately 9 volts at the emitter. I could then use a resistor to knock it down to about 4 volts to light the two LEDs.

Apparently there is a large voltage drop at the transistor resulting in a 2.6 volt output.

• Does anyone know why I only get 2.6 volts coming out of the transistor?
• Any idea how I can correct this to get what I want?

Answer 1

The emitter voltage is 0.7V below the base voltage.

So \$3.3-0.7=2.6V\$. Place the diodes in the collector and a resistor in the emitter as shown. Select the resistor for a current that will give the desired brightness.

For example if 1 mA is desired then \$R_1=2.6/.001=2.6k\Omega\$. The nearest 5% value is \$2.7k\Omega\$.

^{simulate this circuit – Schematic created using CircuitLab}