Fig. 3 shows the receiver circuit. The modulated signal is received by the IR sensor module (TSOP1738) of the receiver section. The negative pulse of the demodu-lated signal from the sensor is inverted by transistor T2 (BC558).
Initially, output pin 3 of the IR module is high at 6V. When the module detects the IR signal, its pin 3 goes from high to low state. As a result, LED2 is forward biased and it glows to indicate that the signal is being received by the sensor.
When LED2 glows, transistor T2 con-ducts as the potential at its base is low.
Thus the transistor conducts when the sensor output goes low.
This inverting action is depicted by the NOT gate in the block diagram. Tran-sistors T2 through T4 form the signal de-tector. This detector is enabled by the posi-tive voltage at the base of transistor T4.
This voltage is given as a pulse by 220pF capacitor C7 only at the rising edge of the clock (Ck) generated by IC4 (NE555).
When transistor T2 conducts, about 6V becomes available at the base of tran-sistor T3 (BC548) and at the collector of transistor T6 (BC548) simultaneously.
Transistor T3 conducts to pull input pin 2 of IC3 (NE555) low. This negative going pulse at pin 2 triggers monostatble multivibrator IC3 to generate a clock pulse of about 1 Hz.
The base of transistor T6 receives the clock pulse from output pin 3 of IC3 and the transistor conducts. Due to the con-duction of transistor T6, IC5 (CD4033) receives the strobe signal at its pin 2.
PARTS LIST Semiconductors:
IC1, IC2,
IC3, IC4 - NE555 timer
IC5 - CD4033 decade counter IC6 - CD4017 decade counter IC7, IC8 - CD4013 dual D-type flip-flop T1 - SK100 pnp transistor T2, T5,
T7-T10 - BC558 pnp transistor T3, T4, T6,
T11-T14 - BC548 npn transistor IR LED1,
IR LED2 - Infrared LED LED1 - Red LED for power-on LED2 - Red LED for signal LED3 - Red LED for clock pulse LED4-LED7 - Red LED for counter output LED8-LED11 - Red LED for appliances 1
through 4 D1 - 1N4148 diode D2-D5 - 1N4001 diode Resistors (all ¼-watt, ±5% carbon, unless stated otherwise):
R1 - 56-ohm
R2 - 820-ohm
R3 - 2.2-kilo-ohm R4, R13 - 47-kilo-ohm R5 - 1-kilo-ohm
R6 - 47-ohm
R7 - 22-ohm
R8 - 470-ohm
R9, R15 - 33-kilo-ohm R10, R11,
R20 - 22-kilo-ohm R12, R22,
R27 - 4.7-kilo-ohm R14 - 1.5-kilo-ohm R16, R21 - 100-kilo-ohm
R17 - 680-ohm
R18 - 150-kilo-ohm R19 - 470-kilo-ohm R23-R26, R29,
R32, R34, - 3.3-kilo-ohm R28, R31, R33,
R35, R36 - 56-kilo-ohm R30, R35 - 220-ohm VR1-VR4 - 10-kilo-ohm preset VR5 - 2.2-kilo-ohm VR6 - 1-mega-ohm Capacitors:
C1, C3, C4, C8, C11-C13, C14,
C15, C16 - 0.01μF ceramic C2, C9 - 1μF, 25V electrolytic C5, C7 - 220pF ceramic C6, C12 - 0.02μF ceramic C10 - 2.2μF, 25V electrolytic Miscellaneous:
S1-S4 - Tactile switch S5 - On/off slide switch RL1-RL4 - 6V, 200-ohm, 1C/O relay Power supply - 9V battery, 6V DC regulated
Fig. 2: Transmitter circuit
Thus transistor T6 acts as a switch between the received signal and the strobe signal at pin 2 of IC5. It allows the demodulated signal to enter the counter section of the circuit for a preset period decided by 1μF capacitor C9 and 1-mega-ohm potmeter VR6. During this pe-riod, the counter ICs (IC5 and IC6) count the input frequency.
IC4 (NE555) is configured in astable mode and functions as a clock generator. It continuously generates a clock pulse of 1.6-second duration at its pin 3. This pulse is simulta-neously fed to pin 1 of IC5 and the bases of transistors T7 through T10 (BC558) via 4.7k resistor R27.
IC5 (CD4033) is a decade counter that provides the re-quired clock pulse to the second counter (IC6). It counts the units and after every ten counts sends a carry-out at its pin 5.
IC6 (CD4017) is also a de-cade counter. It receives the clock from pin 5 of IC5 at its input pin 14. Output pins 2, 4, 7, and 10 of IC6 are connected to the emitters of transistors T7 through T10 via resistors R23 through R26 (each 3.3 kio-ohms), respectively. As Q1 out-put of IC6 is normally high, it is not used. Thus only nine out of possible ten outputs can be used.
Here we’ve used only four out-puts.
When the clock (Ck) goes low, counting stops and only the last high output is passed to the flip-flop circuit through the cor-responding tristate switch (T7, T8, T9, or T10). The collectors of transistors T7 and T8 are connected to pins 3 and 11 of IC7, respectively. IC7 and IC8 each comprises two flip-flops.
The total four flip-flops, namely, IC7(a), IC7(b), IC8 (a), and IC8(b), are wired in toggle mode.
These flip-flops change state for every positive-going pulse ap-pearing at their inputs. The flip-flop outputs are fed to relay driver transistors.
IC7(CD4013) is configured as a latch dual D-type flip-flop.
Fig. 3: Receiver circuit
Fig. 4 :The actual-size, solder-side PCB of the multichannel remote control system comprising transmitter (above) and receiver (below) sections
Fig. 5: Component layout for the PCB
Its output pin 1 is latched when a posi-tive-going transition clock pulse is received at pin 3. Similar is the case at output pin 13 when a positive-going transition pulse is received at pin 11 of this IC. Output pins 1 and 13 of IC7 are connected to relay driver tran-sistors T11 and T12 (BC548) via resis-tors R29 and R32 (each 3.3 kilo-ohms), respectively. The glowing of LED8 and LED9 indicates energisation of relays RL1 and RL2.
The collectors of transistors T9 and T10 (BC558) are fed to pins 3 and 11 of IC8 (CD4013), respectively. Output pins 1 and 13 of IC8 are connected to relay driver transistors T13 and T14 (BC558) via resistors R34 and R36, respectively.
The glowing of LED10 and LED11 indi-cates energisa-tion of relays RL3 and RL4.
LED4 through LED7 connected via re-sistors R23 through R26, respectively, in-dicate the status of the output of IC6. The glowing of LED4 indicates that output pin 2 (Q1) of IC6 is high, which means that the appliance connected through relay RL1 will be turned on or off. Similarly, the glowing of LED5 indicates that out-put pin 4 of IC6 is high and relay RL2 is activated.
Calibration
After all the connections are done, switch on both the transmitter and the receiver.
Place the IR LEDs and the IR sensor facing each other about 10 cm apart.
Now on pressing any of switches S1 through S4 in the transmitter section, LED2 in the receiver section should glow.
During the high input clock pulse (Ck), you’ll observe a light running at the out-puts of IC6. At low input clock pulse, the flip-flop toggles. So you will see one of the LEDs connected to the relay driver (LED8 through LED11) glowing for every posi-tive-going clock (CP).
In the transmitter section, adjust any of presets VR1 through VR4 and press the switch connected in series with it such that the relay connected to the ap-pliance you want to turn on/off gets acti-vated. The relay activation is indicated by the glowing of the corresponding LED (LED8 through LED11). Release the switch once the desired load is turned on/off.
The actual-size, solder-side PCB of the multichannel remote control system com-prising transmitter (above) and receiver (below) sections is shown in Fig. 4 and its
Readers’ comments:
Q1. If any of switches S1 through S4 in the transmitter section is pressed when preset VR6 is at a low resistance, LED2 in the receiver section blinks. But there is no light running through LED4 through LED7 and LED8 through LED11 are always in ‘on’ condition. When preset VR6 is at a high resistance, LED2 glows continuously.
Q2. After I made a small correction in the receiver circuit as shown in Fig. 1 here, I observed light running through LED4 through LED7. But still LED8 through LED11 are always in ‘on’
condition. What could be the reason?
R. Senthil Kumar Ranipet, Tamil Nadu The author, Kaushik Hazarika, replies:
A1. VR6 determines the duration of
timing pulse given out by IC3 (NE555) configured as monostable circuit. This pulse activates transistor T6 (BC548), which is acting as a switching device, and allows the incoming signal to go to
the counters. A very low-output frequency setting will not result in any count as the incoming signal will be blocked during the cut-off period of the transistor (see Fig. 1 in the article). For proper working of the counters, set VR6 such that the output of the monostable is approx. 1 second.
A2. The suggested modification may result in spurious signal count. Simply replace 33k resistor R9 with around 15k resistor, instead. The fault may be due to Hfe variations between different makes of transistors of the same number. The outputs are high due to false triggering of flip-flops caused by the noisy power supply. Use the power-on-reset circuit by disconnecting pins 4 and 10 from GND (see Fig. 1 in the article). The outputs should remain low now. Otherwise, check flip-flops (CD4013).
Fig. 1: Modified circuit of simple multichannel remote control system component layout in Fig. 5. The combined
PCB can be cut along the dotted lines to separate the remote transmitter unit and the receiver unit.
Cautions. 1. The switch (S1, S2, S3, or
S4) must be kept depressed and the trans-mitter oriented towards the receiver sensor until the desired load turns on/off. Release the button during the low period of the clock. This can be easily achieved by
ob-serving the output LEDs (LED8-LED11).
2. It may take 2 to 3 seconds to turn on/off a load. Break of the signal during this period may cause switching of a dif-ferent load. T
BHASKAR BANERJEE