D
uring a conference where speakers are allotted different time slots for completing their speech, it is essential to use a suit- able conference timer which could be programmed for the given time slot. It should not only provide indication as to when the allotted time slot. It should not only provide indication as to when the allotted time is over, but also about the leftover time at any given instant.The conference timer presented here is
tion is preset using tactile switches S2 and S3, with slide switch S1 in ‘set’ position. Once the time duration is preset, the same is displayed in 7-segment LED displays (DIS.1 and DIS.2).
designed to incorporate all such facilities and is expected to prove quite useful. This conference timer is just a 2-digit (minutes) countdown timer which can be preset from 01 minute to 99 minutes. The time dura-
ParTs LisT
Semiconductors:
IC1, IC2 - CD4511B, BCD-to-7-segment latch/decoder/driver
IC3, IC4 - CD4510 BCD up/down coun- ter
IC5 - 4060B 14-stage counter/ di- vider/oscillator
T1-T4 - BC547 npn transistor T5 - SL100 npn transistor D1-D8 - 1N4007 rectifier diode D9 - 5.1V zener
LED1 - Red LED
DIS1, DIS2 - LTS543 common-cathode display
Resistors (all ¼watt, ±5% carbon film, unless stated otherwise) R1-R14 - 470-ohm R15 - 22-kilo-ohm R16-R18 R24, R28, R31 - 100-kilo-ohm R19 - 100 ohm R20, R27 - 33-kilo-ohm R21 - 470-kilo-ohm R22, R23 - 10-kilo-ohm R25, R30 - 2.2-kilo-ohm R26 - 470-ohm R29 - 22-kilo-ohm VR1 - 50-kilo-ohm preset VR2 - 1-mega-ohm preset Capacitors: C1 - 180pF ceramic disk C2 - 47pF, 25V electrolytic C3 - 0.01uF ceramic disk C4 - 0.047uF ceramic disk
Fig. 3: Actual-size, single-sided PCB layout Fig. 4: Component layout for the PCB
As soon as the designated speaker starts speaking, switch S1 is flipped from ‘set’ position to ‘start’ position. The displayed time will start decrementing once a minute until it becomes 00, i.e the unit digit (DIS.1) and tens digit (DIS.2) both become zero. At this juncture, the timer stops decrementing further and an interrupted beep sound is heard from the buzzer, indicating that the time allotted to the particular speaker is over.
In this circuit, IC5 (CD4060B, a 14-stage binary counter with internal oscilla- tor) is used for generation of the basic timing pulses. Presets VR1 and VR2 are required to be adjusted for obtaining ap- proximately 1 Hz (1.0666 Hz, to be more precise) pulses from pin 7 (Q4) of IC5, while the pulses from pin 15 (Q10) are available at the rate of one pulse per minute. For countdown timer operation, this pulse (one-per- minute) is simultaneously ap- plied to pin 15 of IC3 and IC4 through switch S1 (in start posi- tion), and diodes D3 and D4. For presetting this timer, bounceless pulses are required at clock pin 15 of both IC3 and IC4. For this reason 1Hz (bounceless) pulses available from pin 7 of IC5 are used to preset the timer.
IC1 and IC2 (CD4511B), 7-segment latch and driver, accept BCD input code from up/down counters IC3 and IC4 (CD4510B). They convert the BCD code to 7-segment positive logic output code to display the equivalent decimal digits. While displaying decimal digits 9 and 6, their tails are not displayed. The store function available in these ICs is not used in this
circuit and hence the store pin 5 of IC1 and IC2 is made per- manently low.
CD4510B is a divided-by-10 BCD up/down counter. This counter increments or decre- ments by one count for every low-to-high transition of the clock pulse pplied to its clock pin15, depending on the logic level at its pin 10. Thus, when pin 10 of IC3 and IC4 are held high, the counters increment by one count for every clock pulse, and when they are held low, the counters decrement by one count for every clock pulse. The up counting mode is used to preset the conference timer while its down counting mode is used for normal timer operation.
When presetting, the carry ‘out’ pin 7 of IC3 and carry ‘in’ pin 5 of IC4 are not cascaded, to permit presetting of tens and units digits independently (using push-to-on tactile switches S2 and S3, respectively). In countdown mode carry
‘out’ pin 7 of IC3 and carry ‘in’ pin 5 of IC4 are cascaded for 2-digit countdown timer operation. For presetting function, 1Hz (approx.) pulses are used, while for nor- mal cascaded countdown operation of the timer, pulse rate of one-pulse-perminute is used. As stated earlier, these two types of pulses are available from pins 7 and 15, respectively, of IC5.
Transistor T2 is used to stop or activate the IC5 binary counter. When transistor T2 is in ‘cut-off’’ state, its col- lector voltage goes ‘high’. As a result, the positive supply rail is extended to pin 11 of IC5 via resistor R23 and diode D6 to stop IC5 from counting further. When transistor T2 conducts, its collector volt- age goes low and counter IC5 becomes active. The stop and run functions of IC5 binary counter are used during count- down operation only. While presetting, the IC5 binary counter will be in running condition.
When slide switch S1 is slid to ‘set’ posi- tion, pin 10 of both IC3 and IC4 is taken
‘high’ to select the countup mode for preset- ting the timer. At the same time, transistor T12 gets forward biased and conducts. As a result, its collector as well as pin 5 of IC4 go ‘low’. Pin 5 of IC3 is permanently low and both these ICs are not cascaded. The one-pulse-per-minute (from pin 15 of IC5) is no longer available to diode D3-D4 junction, while 1Hz pulse (available from pin 7 of IC5 ) may be applied to the clock input in 15 of IC3 or IC4 by pressing the respective tactile switches S2 and S3. For presetting the timer, depress tactile switch S2 and S3 until desired count is displayed in unit and tens digits (DIS.1 and DIS.2). When desired digit has been displayed in DIS.1 or DIS.2, immediately release switch S2 and S3, as the case may be. Due to conduction of diode D7, transis- tor T2 will be in ‘on’ state and thus binary counter (IC5) is in runing condingion. At the same time, ‘auto reset’ transistor T4 will also be in ‘on’ state, with its collector pulled low. Thus, IC5 will continue to operate normally.
When slide switch S1 is slid from ‘set’ position to ‘start’ position, the red LED1 immediately glows. Transistor T4
goes to ‘cut-off’ and its collector transits from ‘low’ to ‘high’ state. The high-going spike is coupled through capacitor C3 to reset pin 12 of IC5. Thus, IC5 is reset and starts counting from beginning. During this operation, pin 10 of both IC3 and IC4 are held low to select count- down mode of operation. Transistor T1 goes to ‘cut-off’ state. Thus carry ‘out’ pin 7 of IC3 and carry ‘in’ pin 5 of IC4 are cascaded through resistor R16. The one-pulse-per-minute is applied to pin 15 of both ICs (IC3 and IC4) through diodes D3 and D4. Now the digits dis- played in DIS.1-DIS.2 combination start decrementing once every minute. When digits displayed in DIS.1-DIS.2 become ‘00’, carry ‘out’ pin 7 of both IC3 and IC4 goes ‘low’ and transistor T2 does not conduct. As a result, collector of transistor T2 goes ‘high’ and the binary counter stops counting. Simultaneously, transistor T3 conducts and activates the buzzer (functioning in interrupted mode). Thus, interrupted beep sound is heard from the buzzer, indicating that preset time duration has ended. Pin 7 of both IC3 and IC4 goes ‘low’ during display of
digits ‘00’ in DIS.1 and DIS.2. During display of any digits other than ‘00’, the carry ‘out’ pin 7 of either IC3 or IC4 will be high or both may be ‘high’. When the timer is in countdown mode, do not press switch S2 or S3 to avoid disturbance in timer setting.
This circuit, apart from using as a conference timer, may be converted into programmable 2-digit ‘on’ or ‘off’ timer to switch ‘on’/‘off’ any electrical or electronic appliance after 1 minute to 99 minutes duration by incorporating additional add- on circuit shown in Fig.2. During ‘on’/‘off’ timer operation, the digits displayed in DIS.1-DIS.2 help one to know the exact leftover time to switch ‘on’/‘off’ the appli- ance. When using this circuit as ‘off’ timer, slide switch S4 to ‘off’ position and, for ‘on’ timer operation, slide switch S4 to ‘on’ position. When displayed digits become ‘00’, the relay will be energised to turn ‘on’/‘off’ the load.
The actual-size, single-sided PCB for the circuit in Fig.1 is shown in Fig. 3, while its component layout is given in