miteSh P. PariKh
T
his programmable timer is useful for domestic, commercial as well as industrial applications. It auto-matically turns the appliance on/off after apreset time. The time period can be varied from 8 seconds to 2 hours with the help of rotary switches S2 and S3. The circuit works in two modes: off mode and cyclic
mode. Slide switch S4 is used for mode selection.
In the off mode, the appliance turns on after a preset time (set by rotary switch
S2), remains on for another preset time (set by rotary switch S3) and then turns off. In the cyclic mode, this process repeats again and again.
The circuit is built around three quad two-input NAND gate ICs CD4011 (IC1, IC3 and IC5), two 14-bit binary ripple counters CD4020 (IC2 and IC4) and a relay driver transistor (T1). It works off a 12V DC, 500mA power supply. You can also power the circuit from mains by using a 12V DC, 500mA adaptor in place of the 12V DC power supply.
Let’s assume that you want an ap-pliance to turn on after two minutes and
keep it on for another two minutes. For this set the rotary switches S2 and S3 to positions as shown in the figure.
Initially, when power switch S1 is closed, a small charging current pulse through capacitors C2 and C3 resets both the counters (IC2 and IC4) to make all their outputs (Q4 through Q14) low. The high output at pin 10 of NAND gate N3 starts the first oscillator comprising NAND gates N1 and N2, which provides clock pulses to IC2 at the rate of one pulse per second.
The glowing of red LED (LED1) indicates that this oscillator is working well and timer is ‘on.’
During the first 2 minutes, relay RL1 remains de-energised by the control circuit formed by NAND gates N7, N8 and N9 and LED2 is off, which indicates that the appliance is in ‘off’ codition. The second os-cillator built around NAND gates N4 and N5 (which provides clock pulses to IC4 at the rate of one pulse per second) is inhib-ited by the timing control circuit formed by NAND gates N6, N10 and N11.
After 128 pulses (approximately two minutes), the Q8 output of IC2 goes high to perform the following three functions:
1. Make the output at pin 10 of NAND gate N3 low via rotary switch S2, which
inhibits the first oscillator
2. Energise relay RL1 via NAND gates N8 and N9 and relay driver transistor T1 to make appliance ‘on’
3. Make the output at pin 10 of NAND gate N10 low, which is connected to the inputs of NAND gate N11 to make its output at pin 11 high. This high output is further connected to the input (pin 1) of NAND gate N4.
Now the second oscillator starts oscil-lating and provides clock pulses to pin 10 of IC4 at the rate of one pulse per second.
Now, after 128 pulses (approximately two minutes), the Q8 output of IC4 goes
high. This de-energises the relay via NAND gates N7 and N9 and relay driver transistor T1, provided the mode-selection slide switch S4 is towards off position. The high Q8 output will inhibit the second oscillator via NAND gates N6, N10 and N11 to stop clock pulses to pin 10 of IC4.
Thus the relay is energised only once (for 2 minutes) since clock pulses to both IC2 and IC4 are stopped altogether and their outputs get latched.
In case the mode-selector switch S4 is towards ‘cycle on’ side, clock pulses to IC4 would continue and the relay is alter-nately energised and de-energised for two
minutes each. This continues until the circuit is switched off and started again, or the mode-selector switch is slided towards
‘cycle off’ side.
Rotary switch S2 is used for start time selection and rotary switch S3 is used for hold time selection. The start and hold time can be increased up to 24 hours by changing the values of R and C components of the oscillator circuit of first and second oscillator.
For heavier load, use a relay of a higher current rating. The circuit can be made on a multipurpose pCB and put in a plastic or metal cabinet with proper ventilation.
AntI-BAg-snAtChIng AlArm
D. mohan Kumar
H
ere is a simple alarm circuit to thwart snatching of your valu- ables while travelling. The circuit kept in your bag or suitcase sounds a loud alarm, simulating a police horn, if someone attempts to snatch your bag or suitcase. This will draw the attention of other passengers and the burglar can be caught red handed.In the standby mode, the circuit is locked by a plug and socket arrangement (a mono plug with shorted leads plugged
into the mono-jack socket of the unit).
When the burglar tries to snatch the bag, the plug detaches from the unit’s socket to activate the alarm.
The circuit is designed around op-amp IC CA3140 (IC1), which is configured as a comparator. The non-inverting input (pin 3) of IC1 is kept at half the supply voltage (around 4.5V) by the potential divider com-prising resistors R2 and R3 of 100 kilo-ohms each. The inverting input (pin 2) of IC1 is kept low through the shorted plug at the
socket. As a result, the voltage at the non-in-verting input is higher than at the innon-in-verting input and the output of IC1 is high.
The output from pin 6 of IC1 is fed remains low and the alarm is off. Resistor R6, along with capacitor C3 connected to reset pin 4 of IC2, prevents any false Readers’ comments 20A loads, and if any spark develops, is there any method to eliminate spark?
Balakrishnan K. Nair Mumbai The author, Mitesh P. Parikh, replies:
1. I am thankful to Mr Nair for showing interest in my circuit. Here are the replies to his queries:
1. For 12-hour and 24-hour operation, the values of various components are VR1=528 kilo-ohms (using a 1-mega-ohm
preset), R2=0 ohm (R2 os-cillator and negative t e r m i n a l c o n n e c t CD4020), we will get a delay of 12 hours and 24 hours, respectively. you can use a silicon-controlled rectifier (SCR) for switching heavy loads. In this case, there is no spark, but an addition-al circuit is required between the timer and the SCR for triggering and turning off the SCR at the correct time.
Modified circuit of programmable timer for appliances
triggering. Resistor R5 (10 mega-ohms), preset VR (10 mega-ohms) and capacitor C2 (4.7 µF, 16V) are timing components.
With these values, the output at pin 3 of IC2 is about one minute, which can be increased by increasing either the value of capacitor C2 or preset VR.
When there is an attempt at snatching, the plug connected to the circuit detaches.
At that moment, the voltage at the invert-ing input of IC1 exceeds the voltage at the non-inverting input and subsequently its output goes low. This sends a low pulse to trigger pin 2 of IC2 to make its output pin
3 high. Consequently, the alarm circuit built around IC UM3561 (IC3) gets the supply voltage at its pin 5.
IC UM3561 is a complex ROM with an inbuilt oscillator. Resistor R8 forms the oscil-lator component. Its output is fed to the base of single-stage transistor amplifier BD139 (T1) through resistor R9 (1 kilo-ohm).
The alarm tone generated from IC3 is amplified by transistor T1. A loudspeaker is connected to the collector of T1 to pro-duce the alarm. The alarm can be put off if the plug is inserted into the socket again.
Transistor T1 requires a heat-sink.
Resistor R7 (330 ohms) limits the current to IC3 and zener diode ZD1 limits the supply voltage to IC3 to a safe level of 3.3 volts. Resistor R9 limits the current to the base of T1.
The circuit can be easily constructed on a vero board or general-purpose pCB.
Use a small case for housing the circuit and 9V battery. The speaker should be small so as to make the gadget handy.
Connect a thin plastic wire to the plug and secure it in your hand or tie up somewhere else so that when the bag is pulled, the plug detaches from the socket easily.