Automated Alarm Circuits

Two alarm circuits are presented here. One produces bird-chirping sound and the other British police siren tone. Fig. 1 shows the circuit of the birdchirping- sound alarm unit along with the circuit of the control unit. Fig. 2 shows the circuit of only the British police siren tone generator, which has to be integrated with the control circuit portion of Fig. 1 at points A and B to complete the circuit diagram of automated alarm. The control unit is built around ICs CD4047 and CD4027 (as shown on the left side of the dotted line in Fig. 1). As mentioned earlier, it is common to both the alarm circuits. IC CD4047 (IC1) is wired in positive-edge-triggering monostable multivibrator mode to set and reset IC CD4027 (IC2). The output pulse width of IC1 depends on the values of capacitor C2 and resistor R3 connected to its pins 1, 2 and 3.





Normally, when the door is closed, reed switch S1 is closed, transistor T1 conducts and the monostable multivibrator (IC1) remains in standby mode with ‘low’ output at pin 10. When the door is opened, reed witch S1 gets disconnected, T1 stops conducting and low-to-high pulse at pin8 of IC1 triggers the monostable and a short-duration positive pulse of about 10 seconds is available as Q output at pin 10. At the same time, complementary output Q goes low at pin 11. The output from IC1 is used to set and reset IC2. IC2 is a low-power, dual J-K master/ slave flip-flop having independent J, K, set, reset and clock inputs. The flip-flops change states on the positive-going transition of the clock pulses. IC2 is wired such that its Q output turns ‘high’ when reset pin 4 receives a high pulse. When set pin 7 receives a high pulse, Q output goes low and Q output goes high. This lights up LED2 and drives transistor T2 (BC548), which enables the alarm circuit. The output at point A is used to enable the alarm tone generator circuit (on the right side of the dotted line) consisting of two 555 timer ICs marked as IC3 and IC4. The R-C network determines the frequency of the sound produced. The triangular waveform of the astable multivibrator is taken out from the junction of pins 2 and 6 of IC3. This waveform is fed as the control voltage at pin 5 of IC4 through resistor R18. The output received from pin 3 of IC4 is fed to the base of transistor T3 to drive an 8-ohm loudspeaker (LS1), which generates the bird-chirping sound. For the chirping-sound alarm generator, assemble the circuit shown in Fig. 1 on a separate general-purpose PCB and enclose in a small box. And if you want an alarm circuit with British police siren tone, assemble the circuit shown in Fig. 2 on another generalpurpose PCB and connect it to points A and B of the control unit shown in Fig. 1 after removing the circuit on the right side of the dotted line. Use a 9V, 500mA standard adaptor to power the circuit. This circuit may be used as a security alarm in banks, households and motorcars.

Antisleep Alarm for Students

• This circuit saves both time and electricity for students. It helps to prevent them from dozing off while studying, by sounding a beep at a fixed time interval, say, 30 minutes. If the student is awake during the beep, he can reset the circuit to beep in the next 30 minutes.
  
• If the timer is not reset during this time, it means the student is in deep sleep or not in the room, and the circuit switches off the light and fan in the room, thus preventing the wastage of electricity.
  
• The circuit is built around Schmitttrigger NAND gate IC CD4093 (IC1), timer IC CD4020 (IC2), transistors BC547, relay RL1 and buzzer.
  
• The Schmitt-trigger NAND gate (IC1) is configured as an astable multivibrator to generate clock for the timer (IC2).
• The time period can be calculated as T=1.38×R×C.
  
• If R=R1+VR1=15 kilo-ohms and C=C2=10 μF, you’ll get ‘T’ as 0.21 second. Timer IC CD4020 (IC2) is a 14-stage ripple counter.
  
• Around half an hour after the reset of IC1, transistors T1, T2 and T3 drive the buzzer to sound an intermediate beep.
• If IC2 is not reset through S1 at that time, around one minute later the output of gate N4 goes high and transistor T4 conducts.
• As the output of gate N4 is connected to the clock input (pin 10) of IC2 through diode D3, further counting stops and relay RL1 energises to deactivate all the appliances.
  
• This state changes only when IC1 is reset by pressing switch S1. Assemble the circuit on a general purpose PCB and enclose it in a suitable cabinet. Mount switch S1 and the buzzer on the front panel and the relay at the back side of the box.
• Place the 12V battery in the cabinet for powering the circuit. In place of the battery, you can also use a 12V DC adaptor.

ANTI-COLLISION REAR LIGHT

• During poor visibility, i.e., when there is fog, or at dawn or dusk, or when your vehicle gets stalled on a lonely stretch of a highway, this flashing light will provide safety and attract the attention of people to help you out. It uses highbrightness yellow LEDs.
• The circuit uses a dual binary counter CD4520, quadruple 2-input NAND schmitt trigger CD4093, 8-stage shift-and-store bus register CD4094 and some descrete components. An oscillator is built around gate A, whose frequency can be varied through preset VR1 when required.
• The output of the oscillator is fed to IC1 and IC3. When the circuit is switched on, the oscillator starts oscillating, the counter starts counting through IC1 and the data is shifted on positive-going clock through IC3.
• As a result, the four groups of LEDs flash one by one. All the LEDs will then glow for some time and switch off for some time, and the cycle will repeat. Input pins 12 and 13 of the unused gate D must be tied to ground and pin 11 left open.
• Preset VR1 should be of cermet type and used to change the flashing rate of each group of LEDs. The circuit works off regulated 12V. Assemble it on a general-purpose PCB and house suitably.