![]() 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. ![]() ![]() This latter point is of particular importance in security applications in which the circuit is used with several widely separated 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 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.Ī disadvantage of the basic Figure 1 circuit is that it passes the full ‘alarm’ current through the n.o. switches can be wired in parallel, so that the alarm operates when any of these switches are closed. Simple door-bell type close-to-operate alarm circuit. 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 1.įIGURE 1. They can be used to attract attention when someone operates a push switch, or to give a warning when someone opens a door or treads on a pressure pad or tries to steal an item that is wired into a security loop, or to give some type of alarm or safety action when a piece of machinery moves beyond a preset limit and activates a microswitch, etc.Ī wide range of practical contact-operated security circuits are described in this article. The security circuit’s output may take the form of some type of alarm-sound generator, or may take the form of a relay that can activate any external electrical device, and may be designed to give non-latching, self-latching, or one-shot output operation.Ĭontact-operated security systems have many practical applications in the home, in commercial buildings, and in industry. These contacts may take the form of a simple push-button switch, a pressure-pad switch, or a magnetically-activated reed switch, etc. Contact-operated security circuits are units that are activated by the opening or closing of a set of electrical contacts.
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