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U3:C and U3:D form an S-R (Set-Reset) Latch. This is cleared by the Exit Delay timer when the unit is switched on. If the output of U2:B should go high, even momentarily, the latch will change state, triggering the alarm.

The Entry/Exit input is shown in figure 2A. In the normal state, pin 1 of U2:A is held high by the N/C (Normally Closed) sensors, and pin 2 is held high by R12. If a sensor on the N/C circuit should operate, pin 1 will be taken low by R13, while of a N/O (Normally Open) sensor were to operate it would pull pin 2 low. Note that both N/O and N/C sensors may be used simultaneously (providing they are connected correctly), and either will trigger the alarm. C6 and C7 remove any noise which may be picked up along these connections. U3:A and U3:B form a latch which operates in the same manner as that used for the Loop input.

The active-low Loop Only signal on pin 12 of U2:D comes from the key-switch. When the line is low it prevents the signal from the latch from reaching the Entry Timer.

U5:A is the Entry Timer, which is triggered by the negative going output of U2:D. RV1 sets the entry delay period from 5 to 30 seconds. The output on pin 5 of U5:A is high during the entry delay period. U7:B gates the signals from U2:D and U5:A to give an output that is high when the Entry Latch has operated and the Entry Timer has timed out. R20 and C12 provide a very short delay to compensate for the propagation delay of U5:A.

The Alarm Timer and output are shown in figure 1B. When either the Entry Alarm or Loop Alarm line goes high, the output of U7:A goes low, triggering the Alarm Timer (U4). The values of R7 and C3 give a calculated time period of 16 minutes, although in practice this may be a minute or two longer due to the minute leakage current in C3. Even in this case the alarm period will be less than 20 minutes, which is now a legal requirement.

During the alarm period, Q1 is turned on, driving the alarm sounder connected to terminals 1 and 2 of SK2 and the optional relay (RL1:A). D1 protects the transistor from the back-EMF produced by the relay when it is switched off. The relay contacts are bought out to terminals on SK2, together with the circuits 0V rail, for connection to a remote sounder or alarm system if required. If this option is not used, RL1 and R21 should be omitted to conserve battery life.

Gate U7:D is unused, so both inputs are connected to the 9V rail and the output is left unconnected.

The battery (9V PP9 type) connects to SK3 and the key-switch to SK4. When the switch is set so that terminals 1 and 3 of SK4 are linked, power is connected to the circuit via D2 and the active low Loop Only line is taken high via D3. When the switch links terminals 2 and 3, power is connected directly to the circuit and the active low Loop Only line is low.

The Power-On Reset circuit and Exit Timer are shown in figure 2B. When the unit is switched on C15 will be discharged, holding the inputs of U6:A high. C15 will charge via R19 within half a second. The output of U6:A will therefore be low for a brief period when the unit is switched on, giving a suitable signal to trigger the Exit Timer (U5:B), and reset the entry and alarm timers. The device used for U6 has Schmitt trigger inputs to give reliable operation with an input signal that does not conform to digital levels.

U6:D inverts the signal from the Loop input, so that the Exit Timer (U5:B) is held reset and therefore disabled, if the loop is not connected. RV2 sets the Exit period from 5 to 30 seconds (approx).

The Warning Sounder is shown in figure 2C. During the Exit and Entry periods the output of U7:C will be low. This is inverted by U6:C, giving a high level to U6:B. This enables the oscillator formed by U6:B, R18 and C13, which operates at a frequency of about 2Hz. This drives a self-contained audible sounder (AWD1) via Q3, so that the sounder beeps twice every second. For this type of oscillator to operate an IC with a Schmitt trigger input is specified.

The Status LED is shown in figure 1B. The output of U2:C goes high in sympathy with the warning sounder, and also when the alarm has operated. Thus the LED flashes during the Exit and Entry periods, and lights steadily when alarm is (or has been) triggered. In the normal operating state it remains off to conserve battery life.

D8 (4V7 zener) in conjunction with the forward voltage drop of D4 (red LED) act as a basic low battery indicator. The LED brightness will reduce as the battery runs down, and it will not illuminate if the battery voltage is below about 6.5V, although the Warning Sounder will continue to operate. The remainder of the circuit will operate with a battery voltage as low as 6.0V, but prompt battery replacement is recommended when the LED fails to light or becomes very dim.

The circuit consumes under 1mA in the armed state. Most of this is due to U1, U4 and U5. The unit will give typically six months continuous operation from a high capacity 9V battery such as a PP9. A large battery is also more able to operate the alarm sounder.