#032 SwitchNPN - NPN Digital Logic Switch

Use an Arduino to demonstrate a digital switch using an NPN BJT, and plot the results with Processing.

Notes

This is a demonstration a standard small-signal switching circuit using an NPN transistor.

A digital out pin is used to flip the switch by sending the base of the transistor alternately high and low.

The BC547 transistor used here is rated for 100mA max, so only a small (LED) load is connected. Designing for a 2mA current, the current gain is 220 and thus the expected current to the base (Ib) is 9uA.

At Ib=9uA and Vbe=0.7V, desired Rb is:

Rb = (Vin - Vbe) / Ib = (5 - 0.7)/ 9x10^-6 = 478kΩ

Looking at my parts box, I picked 330kΩ instead. Actual current measurements with the switch ON:

Aside from the LED, an analog input pin is used to read a voltage in the load chain to see what is going on and send the data to LEAP#090 PlotNValues (a simple Processing sketch) for plotting over time.

Here’s a sample trace. It also illustrates the impact of the filter capacitor C1 - the first 5 cycles in the plot are without C1, the remainder with C1 in place.

Construction

Credits and References

• SwitchPNP - equivalent circuit using PNP transistors
• The Transistor as a Switch
• BC547 datasheet
• Download Processing