UNNIKRISHNAN P.R.
Fig. 1: Block diagram for the digital controller.
on IC1, an NE555. A fixed voltage, lower than 2/3 Vcc, is fed to pin 6 (threshold) of IC1. If triggered, its out- put stays in the on state as long as its reset pin 4 does not get a negative pulse or the power supply is interrupted. The output goes low when the supply is switched on again. The output of IC1 provides power supply for the second section.
By pressing Sl, a negative pulse is fed to pin 2, and thus it can be used to switch on the other sections. S2 is used to switch off other sections by reset- ting IC1.
When power supply is interrupted, output of IC1 goes low and switches off the other sections. When power sup- ply is resumed, S1 must be pressed to switch on the circuit. By observing LED1, we can know whether the cir- cuit is on or off.
2. Triggering
This section is used to trigger the circuit at the fall of dusk. It is config- ured around IC2, which is working in the monostable mode. A variable volt- age is fed to trigger pin 2 of IC2 via LDR1 and VR1. In the monostable mode of operation, the output of IC2
stays high as long as its trigger input stays below 1/3 Vcc.
At night, the resistance of LDR1 is high, so the voltage at pin 2 is at a low level and hence the output of IC2 goes high at night. The output condition of IC2 can be detected from LED2. The output of IC2 gives power supply to the next three stages.
3. Resetting
The next section is used to reset the two 4017 decade counter ICs used in the counter section. This is also based on a 555 timer (IC3). Its trig- ger pin 2 is connected to capacitor C4 and resistor R7. In the absence of a supply, there is no voltage across the capacitor. When supply is on, IC3 is triggered because voltage across ca- pacitor is below 1/3 Vcc and its out- put goes to a high state. The voltage across capacitor C4 increases, so that the voltage at pin 2 crosses 1/3 Vcc. The time period for which the output stays in the high state depends upon resistor R8 and capacitor C5, and is given by the relationship 1.1 x R8 x C5.
The output of IC3 is given to the reset pin 15 of decade counters IC5
and IC6. These ICs are reset when IC3 is triggered, and as long as the output of IC3 is in the high state, the counters stay in the reset position.
4. Oscillator
This section is used to produce a square wave output. It is based on IC4, an NE555. Its output frequency depends upon resistors R9, R10 and capacitor C7, and is calculated by the equation
1.443 ––––––––––––––––
(R9 + 2 R10) C7
The output frequency is fed to the counter section.
5. Counting
The counter section is used to count the output frequency from the oscillat- ing section. It uses two CMOS CD4017BS ICs. The frequency from oscillating section, IC4, is fed to the input pin of IC5. IC5 works as a fre- quency divider and its output is fed to the input pin of IC6.
The four outputs of IC6 are com- bined by using diodes D1 through D4. This combined output is given to the last section. If any of the outputs is high, then the load is in ‘on’ state. The condition of this combined out- put is detected by LED3.
6. Output
The output section is used to switch the relay on and off. It is based on BEL187 transistor Tl. The relay is con- nected at the collector of transistor Tl. Diode D5 is connected to protect tran- sistor T1. The output from the count- ing section is fed to the base of T1 through resistor Rl1. If the output from counting section is high, it biases the transistor and thus the relay is acti-
Capacitors: Cl,C3,C6, C8 — 0.0lμF ceramic disc C2 — 100μ, 16V electrolytic C4 — 47nF ceramic C5 — 470μF, 16V electrolytic C7 — l000μF,16V electrolytic C9 — l00μF,25V electrolytic Cl0 — 10μF,16V electrolytic Miscellaneous: RL1 — 9V, 200-ohm, SPST relay S1,S2 — Pushbutton type switch PARTS LIST vated.
Working
By pressing S1, the triggering sec- tion gets the power supply. At the fall of dusk, the resistance of LDR1 in- creases, so a large voltage is dropped across it, resulting in a decrease in the voltage at pin 2.
When the voltage at pin 2 drops
Fig.2: Circuit diagram for the digital controller.
Semiconductors:
IC1 - IC4 — NE555 timer IC5, IC6 — CD4017B decade
counter T1 — BEL187 npn
transistor D1 - D5 — 1N4001 rectifier
dicode
Resistors (all 1/4 watt, ±5% carbon unless stated otherwise): Rl, R13 — 10-kilohm R2 — 56-kilohm R3 — 27-kilohm R4, R6, R12, R14 — 680-ohm R5 — 100-kilohm R7 — 2.2-megohm R8 — 470-kilohm R9 — 120-kilohm RI0 — 330-kilohm R11 — 15-kilohm VR1 — 470-kilohm, variable LDR1 — Light dependent resistor
below 1/3 Vcc, IC2 is triggered and stays in the high state till morning. Since the output of IC2 is high, the next three stages get power supply.
When C4 is fully charged, IC3 is triggered and so the reset pins of IC5 and IC6 both get a positive pulse. Therefore, the first outputs of both ICs are high. This condition changes when the voltage across capacitor C5 crosses 2/3 Vcc, because then the output of IC3 goes to a low state and the reset pins of IC5 and IC6 get a negative pulse each. In this condition, IC5 gets ready to accept pulses from the oscil- lating section and its outputs are changed.
For each set of ten pulses from IC4, the outputs of IC6 are changed one by one. The output time period of oscillat- ing section is about 540.5 seconds, i.e.
about nine minutes. Since IC5 divides the output frequency from IC4, IC6 gets a pulse of time period 9 x 10 = 90 minutes, i.e. one and a half hours. So, the outputs of IC6 are changed every one and a half hours duration.
When the controller is switched on, the output Q0 at pin 3 of IC6 is high and it turns on the load for first one and a half hour. Then, the next output Q1 at pin 2 is high, which switches on the load for the second one and a half hour. Next, output Q2 at pin 4 goes high, which switches on the load for the same period again.
When the output Q3 at pin 7 is high, it switches off the relay as pin 7 has no connection. Then, the output of IC6 changes to Q4, Q5, Q6 for each one and a half hour, and when it reaches Q7 (i.e. pin 6) the load is again switched
on. After one and a half hour, the output is changed to Q8 (pin 9). Since pin 9 of IC6 has no connec- tion, it again switches off the relay. At the break of dawn, the re- sistance of LDR1 decreases, and the trigger pin of IC2 gets a posi- tive voltage greater than 1/3 Vcc, so that its output goes low which, in turn, cuts off power supply to the next three stages.
Assembly
The circuit may be assembled on a general-purpose PCB or the PCB whose pattern is shown in Fig. 3. Front panel can be made according to one’s choice. A model is shown in Fig. 5.
A 9V power supply can be as- sembled, using a step-down transformer and a rectifier, and connected to the cir- cuit as shown in Fig. 2. Relay connec- tions are also shown in Fig. 2.
The front panel bears various con- trols and indication LEDs. In the pres- ence of power, LED1 glows. On press- ing S1, LED4 will glow.
At day time, adjust VR1 to set the trigger LED2 to off position by trial and error method.
Under normal conditions, only LED1 and LED4 will glow. After dusk, trigger LED2 and load LED3 will also glow. After four and a half hours, the load and load LED3 are off. The load LED3 is again activated after six hours, to be turned off after another one and a half hours.
To turn the system off, press S2. Fig. 3: Actual-size PCB layout for the digital controller.
Fig. 4: Component layout for the PCB shown in Fig. 3.
V
OX or voice operated trans- receive control is very conve- nient in operating a transmit- ter, especially in SSB mode. Commer- cially available units are expensive. Most of the available circuits in Ham journals use complex circuitry, which either is highly expensive or most of the components are not easily avail- able in the Indian market. The unit de- scribed here is based on compact and straightforward design, using easily available discrete devices, and is quite inexpensive. The unit is ideal to use with almost any commercial or home- brewed VHF or HF trans-receiver that does not have the facility of VOX op- eration. Further, the performance of thisunit is an improvement over the previ- ously available designs.