JUNOMON ABRAHAM
T
elephone remote control implies control of devices at a remote location via a circuit interfaced to the remote telephone line/device by dial-ing specific DTMF (dual-tone multi-fre-quency) digits from a local telephone. The telephone remote control system described here has the following features:1. It can control multiple channels/
relays.
2. It provides you feedback when the circuit is in energised state and also sends an acknowledgement indicating action w.r.t. the switching ‘on’ of each requested relay and switching ‘off’ of all relays (to-gether).
3. It can selectively switch ‘on’ any one or more relays one after the other and switch ‘off’ all relays simultaneously.
Operation
Instead of straightway proceeding with the circuit description, we shall start with the operation as this would help us in understanding the circuit better. The op-eration is as follows:
1. From the local telephone, dial the
number of the remote tele-phone to which the cir-cuit is con-nected. In a short while you will hear a musical note indicat-ing that the circuit con-nected to the remote tele-phone is ac-tive.
2. Now if you want to switch ‘on’ a particular fol-lowed by any one of digits 1 to 7 corre-sponding to the device/re-lay number that you de-sire to switch
‘on’. The switching ‘on’
of the relay will be ac-knowledged/
indicated by a musical note.
Now you may keep the
the relays, press ‘*’ and then press key for digit 8. A musical note is heard, which indicates that all the relays have been switched ‘off’. Keep the handset on cradle.
The circuit
At the remote telephone end, the ringing signal is detected by a high-input-imped-SUNIL KUMAR
Fig. 1: Schematic diagram of the telephone remote control
open-collector type, the output pin has been pulled towards Vcc via 10-kilo-ohm ance op-amp CA3140E that is wired as a
comparator. Since this op-amp output is
resistor R21. IC2 (NE556) com-prises two timers (NE555 type) that have been configured as monostables.
When a ring is detected by IC1, its output triggers one of the tim-ers in IC 556. The output of the timer after inversion by one of the NAND gates of IC3 (CD4011), en-ables IC4 (CD4060) by taking its reset pin 12 ‘low’. (IC4 is an oscil-lator-cum-14-bit binary counter.) As a result, IC4 starts counting when the ring signal strikes the input of the circuit.
After some time, decided by the setting of preset VR3, Q12 output of IC4 goes ‘high’. This output coupled to pin 8 of a NAND gate inside IC3 will enable it. The de-tected ring signal (if the ring sig-nal is still persisting) applied to pin 9 of the same NAND gate (after inversion by another NAND gate) will pass through it to trigger the second monostable inside IC2 (NE556) as well as IC5 (NE555), which is again wired as a monostable. This arrangement avoids the circuit from being trig-gered by any transients or false ring signals on the telephone line.
The output of the second monostable of IC2, available at its pin 9, drives transistor T2 and shunts the telephone line with 220-ohm resistor (R20). As a result, the telephone line voltage drops to around 10 to 12 volts. This is equivalent to the lifting of the phone handset of the remote tele-phone. As mentioned earlier, both IC5 and the second monostable of IC2 are triggered simultaneously.
The output of monostable IC5 starts melody generator IC6 (UM66) and the musical note obtained from it is coupled to the telephone line.
This informs the caller that the re-mote circuit is in energised state.
As the remote circuit is in energised condition, the next step for the operator Fig. 2: Actualsize, single-sided PCB for the circuit
Fig. 3: Component layout for the PCB
PARTS LIST Semiconductors:
IC1 - CA3140E op-amp
IC2 - NE556 dual timer IC3 - CD4011 quad NAND gate IC4 - CD4060 14-stage counter/
oscillator IC5 - NE555 timer
IC6 - UM66 melody generator IC7 - CM8870 DTMF-decoder IC8 - CD4099 8-bit addressable
latch
IC9 - 7805 regulator +5V T1 - BC548 npn transistor T2-T9 - BC547 npn transistor (only
T2 and T6 shown) LED1, LED2 - Green LED LED3 - Yellow LED
LED4 - Red LED
D1, D2 - 1N4148 switching diode D3-D10 - 1N4007 rectifier diode (only
D3 and D4 shown) Resistors (all ¼-watt, ±5% carbon, unless otherwise stated)
R1, R16, R17 - 150-kilo-ohm R2, R21 - 10-kilo-ohm
R3 - 33-kilo-ohm
R4 - 680-kilo-ohm
R5 - 560-ohm
R6, R10 - 22-kilo-ohm
R7 - 1-mega-ohm
R8, R15 - 390-ohm R9, R12 - 15-kilo-ohm
R11 - 270-ohm
R13, R14 - 3.3k-kilo-ohm R18 - 330-kilo-ohm
R19, R22-R27 - 4.7-kilo-ohm (R22-R27 not shown in the figure)
R20 - 220-ohm
VR1 - 10-kilo-ohm preset VR2 - 1-mega-ohm preset VR3 - 220-kilo-ohm preset VR4 - 470-kilo-ohm preset Capacitor:
C1 - 0.22µF ceramic disk C2 - 220µF, 10V electrolytic C3 - 100µF, 10V electrolytic C4, C5, C8 - 0.01µF ceramic disk C6, C11, C12 - 0.1µF ceramic disk C7 - 10µF, 10V electrolytic C9 - 0.02µF ceramic disk C10 - 0.47µF, 100V polyester Miscellaneous:
XTAL - 3.58MHz crystal RL1-RL7 - 6V, 150-ohm 1C/O relay
(only RL4 shown)
at local telephone is to press the ‘*’ but-ton, which makes the local telephone to operate in the tone-dialing mode. The dig-its that are pressed after pressing the ‘*’
button are converted to DTMF tones.
The tone is decoded by IC7 and its three LSBs (covering binary equivalent of decimal digits 0 through 7) are con-nected to the address inputs, while the MSB line is connected to reset pin 2 of IC8 (CD4099, an 8-bit addressable latch).
When a valid DTMF tone is detected at the input of IC7, its pin 15 goes ‘high’ to enable IC8 after inversion by NAND gate of IC2. At the same time, it triggers IC5 for informing the caller that his key-press is accepted.
Numbers 1 to 7 on the local keypad cause latching of the corresponding re-lays, while number 8 causes reset opera-tion, which means that we can switch ‘on’
seven relays independently one by one and switch ‘off’ all relays simultaneously by pressing number 8. The output of IC8 drives the relays via the relay driver tran-sistor. Truth tables I(A) and I(B) of CD4099 indicate relay operation.
Alignment
1. Connect the circuit to the telephone line.
2. Adjust preset VR1 so that the ring-ing pulse causes LED1 to flicker. For bet-ter performance, set the voltage at pin 3 of IC1 at approximately 2 volts.
3. The time required to activate/
energise the circuit is adjusted by preset VR3 with the help of LED2.
4. The time available for remote switching action can be set by preset VR2 with the help of LED4. Indirectly, the set-ting of preset VR2 determines the charge that will have to be paid to the telecom department.
5. The period of the musical note can be controlled by the adjustment of VR4 with the help of LED3. ❏
RUPANJANA