Close-to-operate circuits

The simplest type of contact-operated security circuit consists of an alarm bell (or a buzzer or electronic ‘siren-sound’ generator, etc.) wired in series with a normally-open (n.o.) close-to-operate switch, the combination being wired across a suitable battery supply, as shown in the basic ‘door-bell’ alarm circuit of Figure 2.1. Note that any desired number of n.o. switches can be wired in parallel, so that the alarm operates when any of these switches are closed. This type of circuit gives an inherently non-latching type of operation, and has the great advantage of drawing zero standby current from its supply battery.

A disadvantage of the basic Figure 2.1 circuit is that it passes the full ‘alarm’ current through the n.o. operating switches and their wiring, so the switches must be fairly robust types, and the wiring must be kept fairly short if excessive wiring volt-drops are to be avoided. This latter point is of particular importance in security applications in which the circuit is used with several widely separated n.o. switches. The solution to this problem is to activate the bell via a ‘slave’ device (which is fitted close to the bell but requires a fairly low input current), and to activate this slave device (and thus the bell) via the security switches. Figures 2.2 to 2.6 show a variety of such circuits, in which the slave device takes the form of a relay, a power transistor, or an SCR.

Figure 2.2 shows a relay-aided version of the close-to-operate alarm circuit. Here, the parallel-connected n.o. switches are wired in series with the coil of a 6V relay (which typically draws an operating current of less than 100mA), and the relay contacts (which can typically switch currents of several amps) are wired in series with the alarm bell, and both combinations are wired across the same 6V supply. Thus, when the switches are open the relay is off and its contacts are open, so the bell is off, but when any one or more of the switches is closed the relay is driven on and its contacts close and activate the alarm bell. Note in the latter case that the switches and their wiring pass a current equal to that of the relay coil; the switches can thus be fairly delicate ones, such as sensitive reed types, and the wiring can be reasonably long. Silicon diode D1 is wired across the relay’s coil to protect the switches against damage from the coil’s switch-off back-emf.

The Figure 2.2 circuit gives a non-latching form of operation, in which the alarm operates only while one or more of the operating switches is closed. In most high-security applications, the circuit should be a self-latching type in which the relay and alarm automatically lock on as soon as any one of the n.o. switches is closed, and can only be de-activated via a security key. Figure 2.3 shows the above circuit modified to give this type of operation. Here, the relay has two sets of n.o. contacts, and one of these is wired in parallel with the n.o. switches so that the relay self-latches as soon as it is operated, and the entire circuit can be enabled or disabled/de-activated via key-switch S1, which is wired in series with the battery supply line. Circuits of this basic type are usually used in low-cost ‘zone protection’ applications, in which the ‘zone’ is a large room or shop floor, the S1 key-switch is located outside of the zone, and the n.o. trigger switches are hidden pressure mat switches or door- or window-operated microswitches fitted within the protected zone.

An alternative solution to the Figure 2.1 switch-and-wiring ‘current’ problem – but which can only be used in non-latching applications – is shown in Figure 2.4, in which npn power transistor Q1 is used as the slave device. Resistor R1 ensures that when any of the activating switches are closed, Q1’s drive current is limited to less than 60mA, which (assuming that Q1 has a nominal current gain of at least ×25) enables the transistor to switch at least 1.5A through the alarm bell.

Another solution to the ‘current’ problem is to use an SCR (silicon controlled rectifier) as the slave device, as shown in Figures 2.5 and 2.6. These circuits rely on the fact that ordinary electromagnetic alarm bells are self-interrupting solenoid devices that incorporate a self-activating on/off switch in series with the solenoid’s supply line. This switch is normally closed, allowing current to reach the solenoid and throw out a striker that hits the bell dome and simultaneously opens the switch, thus breaking the current feed and causing the striker to fall back again until the switch closes again, at which point the whole process starts to repeat, and so on; the bell’s operating current is thus drawn in pulsed form.

In the Figure 2.5 circuit the alarm bell is wired in series with an SCR that has its gate current derived from the positive supply line via current-limiting resistor R1 and via the parallel-connected n.o. security switches, which (when R1 has a value of 1k0) pass operating currents of only a few milliamps. When all the switches are open, the SCR and alarm bell are off, but when any one of the switches is closed it feeds gate current to the SCR via R1, so the SCR turns on and activates the bell. Note in this design that, since the bell is a self-interrupting device, the circuit effectively gives a non-latching type of operation in which the SCR and bell only operate while one or more of the switches are closed.

Figure 2.6 shows how the above circuit can be modified to give self-latching operation. SCRs are inherently self-latching devices that, once they have been initially turned on, remain on until their anode current falls below a ‘minimum holding’ value, at which point the SCR unlatches and turns off. In the Figure 2.5 circuit the SCR thus automatically unlatches each time the alarm bell self-interrupts, but in the modified Figure 2.6 design the bell is shunted via R3, which is wired in series with n.c. switch S4, which ensure that the SCR’s anode current does not fall below the C106’s minimum holding current value when the bell self-interrupts, thus providing the circuit with a self-latching action.

Note that the C106 SCR used in the Figure 2.5 and 2.6 circuits has an anode current rating of only 2A, so the alarm bell must be selected with this point in mind. Alternatively, SCRs with higher current ratings can be used in place of the C106, but this modification will probably necessitate changes in the R1 and R3 values of the circuits. Also note in these SCR circuits that, to compensate for the SCR’s typical 1V anode-to-cathode volt drop, the supply voltage must be at least 1V greater than the nominal operating voltage of the alarm bell.

Open-to-operate circuits

A major weakness of the Figure 2.1 to 2.6 circuits is that they do not give a ‘fail-safe’ form of operation, and give no indication of a fault condition if a break occurs in the contact-switch wiring. This snag is overcome in circuits that are designed to be activated via normally-closed (n.c.) switches, and a basic circuit of this type is shown in Figure 2.7.

In Figure 2.7, the coil of a 12V relay is wired in series with the collector of transistor Q1, and bias resistor R1 is wired between the positive supply line and Q1 base. The alarm bell is wired across the supply lines via n.o. relay contacts RLA/1, and n.c. operating switch S1 (which may consist of any desired number of n.c. switches wired in series) is wired between the base and emitter of the transistor. Thus, when S1 is closed it shorts the base and emitter of Q1 together, so Q1 is cut off and the relay and the bell are inoperative. Under this condition the circuit draws a quiescent current of 1mA via R1. When S1 opens or a break occurs in its wiring, Q1’s base-to-emitter short is removed and the transistor is driven to saturation via R1, thus turning the relay on and activating the alarm bell via relay contacts RLA/1. This basic circuit gives a non-latching type of alarm operation, but can be made to give self-latching operation by wiring a spare set of n.o. relay contacts (RLA/2) between the collector and emitter of Q1, as shown dotted in the diagram.

Thus, the Figure 2.7 circuit gives fail-safe operation, but draws a quiescent or standby current of 1mA. This standby current can be reduced to a mere 25μA by modifying the circuit in the way shown in Figure 2.8. Here, the value of R1 is increased to 470k, and Q1 is used to activate the relay via pnp transistor Q2, and the circuit’s action is such that Q1–Q2 and the relay and bell are all off when S1 is closed, but turn on when S1 is open. The basic circuit gives a non-latching form of operation, but can be made self-latching by wiring a spare set of n.o. relay contacts (RLA/2) between the collector and emitter of Q2, as shown dotted in the diagram.

If desired, the standby current of the Figure 2.8 circuit can be reduced to a mere 1μA or so by using an inverter-connected CMOS gate in place of Q1, as shown in Figure 2.9. The gate used here is taken from a 4001B quad 2-input NOR gate IC, and the three unused gates are disabled by shorting their inputs to the 0V line, as shown in the diagram. The used gate has a near-infinite input impedance, and the standby current of the circuit is determined mainly by the R1 value and by the leakage current of Q1. The basic circuit gives a non-latching form of operation, but can be made self-latching by wiring a spare set of n.o. relay contacts (RLA/2) between the collector and emitter of Q1, as shown dotted in the diagram.

Figure 2.10 shows an alternative way of making the basic Figure 2.8 circuit give self-latching operation, without resorting to the use of a spare set of n.o. relay contacts. In this case the relay-driving transistor (Q1) is driven by a pair of 4001B CMOS NOR gates that are configured as a bistable multivibrator and has an output that goes low and self-latches if S1 is briefly opened or its leads are broken. As the bistable output goes low it turns Q1 on, thus activating the relay and alarm bell. Once the bistable has latched the bell into the ‘on’ state, it can be reset into the standby or ‘off’ mode by closing S1 and momentarily operating RESET switch S2, at which point the bistable’s output latches back into the high state and turns off Q1 and the relay and bell. The circuit draws a quiescent current of about 1μA.

‘Loop’ alarm circuits

One type of contact-operated alarm circuit that is widely used in large shops and stores (and also in domestic garages and garden sheds) is the so-called ‘loop’ alarm, in which a long length of wire is run out from the alarm unit, is looped through a whole string of ‘to be protected’ items in such a way that none of them can be removed without cutting or removing the wire, and is then looped back to the alarm unit again, to complete a closed electrical circuit. The alarm sounds instantly if an attempt is made to steal any of the protected items by cutting the wire loop, i.e. by effectively opening its ‘contacts’. Figure 2.11 shows the circuit of a simple battery-powered unit of this type.

The simple Figure 2.11 loop alarm circuit is a modified version of the self-latching CMOS-aided Figure 2.9 circuit, with its series-connected S1 security switches replaced by a number of series-connected wire ‘loops’ that – when key-operated switch S1 is closed – activate the self-latching alarm if any part of the loop wiring becomes open circuit. In the diagram, only two loops are shown, but in practice any desired number of loops can be used. The entire circuit (except the loops) is housed inside a metal security case, and the loops are connected to screw terminals on the main circuit board via grommet holes in the side of the case; unwanted loops can be replaced by short circuits connected between the appropriate screw terminals. The entire circuit can be turned on and off via key-switch S1.

Figure 2.12 shows an improved version of the Figure 2.11 self-latching loop alarm circuit. The first points to note about this version of the circuit are that a LED is connected across the relay coil via R4 and thus illuminates and gives a visual indication whenever the relay is turned on, and that the circuit’s +12V power feed is controlled via 4-way key-switch S1 and diodes D2 and D3. When S1 is in position ‘17’, the entire circuit is turned off. When S1 is in position ‘2’, the main part of the circuit (including the LED indicator) is active but the alarm bell and self-latching facility are disabled; this TEST (non-latch) position is meant to be used when testing the loop wiring. When S1 is in the position ‘3’ TEST (latching) position, all of the circuit except the bell is enabled. When S1 is in the position ‘4’ ON position, the entire circuit (including the alarm bell) is enabled, and the circuit gives normal ‘security’ operation.

The final point to note about the Figure 2.12 circuit is that n.c. anti-tamper switch S2 is wired in series with the loop network and (when S1 is set to the ON position) activates the self-latching alarm if it (S2) takes up an ‘open’ state. S2 is actually an ordinary n.o. tactile ‘key-pad’ switch with a short coil-spring bonded vertically to its touch-pad, and is fixed to the main circuit board in such a way that the switch is held in the closed n.c. position (via the spring) when the circuit’s security case is closed, but opens (thus sounding the alarm) if the case is opened while the alarm system is still turned on. Anti-tamper switches of this basic type are quite easy to make from readily-available components; Figure 2.13 illustrates the basic method of construction.

Before leaving this Bell and relay-output circuits section of this chapter, note that the various relay-output circuits shown in Figures 2.2, 2.3, and 2.7 to 2.11 can, if desired, be used to activate any type of electrical or electronic alarm or system via their n.o. relay contacts when the relay is triggered in response to an input contact-switching action, and are thus not restricted to use with alarm bells only.