Make: Electronics by Charles Platt (read me a book .TXT) 📕

shows one more component, which you have not seen before: a diode, labeled D1. See Figure 3-93. The diode looks like the heart of an LED, and indeed, that’s pretty much what it is, although some diodes are much more robust. It allows electricity to flow in only one direction, 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 (normally open) leg of the relay via a 10k resistor to the base connection. When the relay is not energized, 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.

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

Essentials

All about diodes

A diode is a very early type of semiconductor. It allows electricity to flow in one direction, but blocks it in the opposite direction. (A light-emitting diode is a much more recent invention.) Like an LED, a diode can be damaged by reversing the voltage and applying excessive power, but most diodes generally have a much greater tolerance for this than LEDs. The end of the diode that blocks positive voltage is always marked, usually with a circular band, while the other end remains unmarked. Diodes are especially useful in logic circuits, and can also convert alternating current (AC) into direct current (DC).

A Zener diode is a special type that we won’t be using in this book. It blocks current completely in one direction, and also blocks it in the other direction until a threshold voltage is reached—much like a PUT.

Signal diodes are available for various different voltages and wattages. The 1N4001 diode that I recommend for the alarm activation circuit is capable of handling a much greater load at a much higher voltage, but I used it because it has a low internal resistance. I wanted the diode to impose a minimal voltage drop, so that the relay would receive as much voltage as possible.

It’s good practice to use diodes at less than their rated capacity. Like any semiconductor, they can overheat and burn out if they are subjected to mistreatment.

The schematic symbol for a diode has only one significant variant: sometimes the triangle is outlined instead of filled solid black (see Figure 3-94).

Figure 3-94. Either of these schematic symbols may be used to represent a diode, but the one on the right is more common than the one on the left.

Completing the Breadboard Alarm Circuit

It’s time now to breadboard the control circuit for your alarm noisemaker. Figure 3-95 shows how this can be done. I am assuming that you still have the noisemaker, which functions as before. I’m assuming that you still have its relevant components mounted on the top half of the breadboard. To save space, I’m just going to show the additional components mounted on the bottom half of the same breadboard.

It’s important to remember that you are not supplying power directly to the left and right “rails” on the breadboard anymore; you are supplying power to the relay-transistor section, and when the relay closes its contacts, the relay supplies power to the rails. These then feed the power up to the top half of the breadboard. So disconnect your power supply from the breadboard rails and reconnect it as shown in Figure 3-95.

Figure 3-95. The schematic that was developed in the previous pages can be emulated with components on a breadboard, as shown here. S1 is a DPDT relay. Wires to the sensor switch network and to the power supply must be added where shown.

Because it’s a double-pole relay, I am using it to switch negative as well as positive. This means that when the relay contacts are open, the noisemaking section of the circuit is completely isolated from the rest of the world.

The breadboarded relay circuit is exactly the same as the schematic in Figure 3-93. The components have just been rearranged and squeezed together so that they will fit alongside the relay. Two wires at the lower-left corner go to the network of magnetic sensor switches that will trip the alarm; for testing purposes, you can just hold the stripped ends of these two wires together to simulate all the switches being closed, and separate the wires to simulate a switch opening.

Two more wires bring power to the breadboard on either side of the relay. This is where you should connect your power supply during testing. The output from the relay, through its top pair of contacts, is connected with the rails of the breadboard by a little jumper wire at top left, and another at top right. Don’t forget to include them! One more little wire at the lower-left corner (easily overlooked) connects the lefthand side rail to the lefthand coil terminal of the relay, so that when the relay is powering the noisemaker circuit, it powers itself as well.

When you mount the diode, remember that the end of it that is marked with a band around it is the end that