Make Electronics

Getting Somewhat More Serious 133

Experiment 15: Intrusion Alarm Revisited

from positive to negative, as shown by its arrow symbol. If current tries to flow
in the opposite direction, the diode blocks it. The only price you pay for this
service is that the diode imposes a small voltage drop on electricity flowing in
the “OK” direction.

So now, positive flow can pass from the transistor, through the diode, to the
relay coil, to get things started. The relay then supplies itself with power, but
the diode prevents the positive voltage from getting back into the transistor
the wrong way.

Perhaps a more elegant solution to the problem is to connect the NO leg of
the relay via a 10k resistor to the base connection. When the relay is not ener-
gized, the NO leg is inert and simply behaves as a parasitic capacitance on the
node. When the relay becomes energized, the NO leg shunts +12V through
the common terminal via a 10k resistor into the base of the transistor. In this
circuit configuration, the transistor is never exposed to a potentially harmful
voltage and you are not depending on leakage currents of non-ideal elements
to protect devices.

However, I needed an opportunity to introduce you to the concept of diodes.
You can check the following section “Essentials: All about diodes” to learn
more.

Q1

D1

R1

1K

10K

Power to

noise

maker

12V

DC

Switches

activated

by opened

doors or

windows

Figure 3-93. Diode D1 has been added to protect the emitter of Q1 from positive voltage
when the relay is energized.