The heart of the source stage is an infrared (IR)-LED. The source is used to emit infrared light, using a small DC voltage since the IR-LED requires a very small current (about few mA). Thus, series resistor Rl is used as a current limiting resistor.
The main part of the detector stage
is the phototransistor. Although the IR light beam is invisible, it behaves like ordinary light and is focused on the phototransistor by using a convex lens. When IR light beam is focused on phototransistor Tl, the resistance of T1 becomes low. Hence, the voltage across resistor R2 increases. An op-amp (IC1) is used in the circuit as a comparator.
When the IR light stops falling on phototransistor Tl, the resistance of phototransistor becomes very high. In this state, Tl and R2 act as a resistive voltage divider at the inverting termi- nal of the comparator (IC1). With re- sistance of Tl being very high, the volt- age at inverting terminal is very low. Using variable resistance VR1 (preset), the voltage at non-inverting terminal of comparator is so adjusted that it is slightly greater than the voltage at in- verting terminal. As a result, the output of IC1 becomes high.
Similarly, the output of ICl becomes low when voltage at its non-inverting terminal is slightly lesser than voltage at the inverting terminal. The comparator’s output changes from high to low or from low to high, with small changes of a few millivolts at its input terminals.
When output of IC1 becomes high, it gives sufficient base bias to drive npn transistor T2 to conduction through diode Dl and resistor R3. Since Dl is forward biased, it conducts only when output of IC1 is high. Resistor R4 keeps transistor T2 ‘off’ when output of IC1 goes low.
When the IR radiation is incidental on transistor Tl, its resistance decreases. This increases the voltage drop across resistor R2 sufficiently to change the output of IC1 from high to low state, and so transistor T2 turns ‘off’. Thus, the collector of T2 becomes more posi- Fig.2: Circuit diagram for the interruption counter cum burglar alarm.
Inside view of author’s prototype tive through resistor R5. No current
flows through LED2 and it remains ‘off’. When LED2 turns ‘on’, it indi- cates that the IR beam has been inter- rupted.
When IR beam is interrupted mo- mentarily, as explained earlier, the out- put of IC1 becomes high and transistor T2 starts conducting. Thus, collector of T2 is grounded for a moment, LED2 lights up momentarily and a clock pulse is produced at point ‘C’. Each time the IR light is interrupted, another clock pulse (square shaped) is produced at the collector of transistor T2.
When selector switch S1 is at posi- tion ‘A’, the pulses produced at collec- tor of T2 indicate the number of inter- ruptions through the counter chain for counting and displaying the actual count digitally.
The first decade counter IC4 pro- cesses the pulses and gives a binary output to IC6 which drives a 7-seg-
ment, common-anode display and shows the corresponding decimal num- ber. After counting up to 9, the tenth pulse overflows to IC3, whose output goes to IC5 that drives another 7-seg- ment display. The two displays together enable a count of up to 99. On 100th
pulse, the combined display shows ‘00’. The pushbutton switch S2 resets the circuit and returns the display to ‘00’. Hence, the maximum number of interruptions that can be counted is 10n-1, where ‘n’ is number of digits.
So, in this case the number is 102-1, i.e.
99.
The number of digits can be in- creased by cascading more counters with decoder drivers and displays, to show a larger number of interruptions. When selector switch Sl is moved to ‘B’, capacitor Cl being initially dis- Fig. 4: Component layout for the PCB shown in Fig. 3.
charged, trigger pin 2 of IC2 gets Vcc through resistor R6. When the light beam is interrupted by someone, the output of IC1 becomes high and pro- duces a pulse at the collector of T2. This pulse triggers IC2 through capaci- tor C1 and makes its output high. This output is given directly to the ion- buzzer which produces a pleasant alarm sound. After a preset interval, the out-
put of IC2 goes low and the buzzer stops producing sound. If another inter- ruption takes place, the alarm again rings for a few seconds and stops automatically, ready for the next inter- ruption.
The current consumption of the alarm circuit being only a few milli- amperes, IC2 can easily withstand this load. As use of relay at the output of
IC2 has been eliminated, the circuit’s overall cost gets reduced.
Power supply
Power supply for the ciruit is very simple as it requires only +5 volts. It uses step-down transformer X1 to re- Fig. 7: Wiring diagram.
Fig. 8: IR-LED and phototransistor arrangement.
PARTS LIST Semiconductors:
IC1 — CA741 op-amp IC2 — NE555 timer
IC3,IC4 — 74LS90 decade counter IC5,IC6 — 74LS247 BCD to 7-
segment decoder display driver
IC7 — LM7805, +5V voltage regulator
T1 — TIL81 phototransistor T2 — BC148 npn transistor LED1 — Infrared light emitting
diode
LED3,LED4 — 5mm general-purpose LED D1 — IN4148 switching diode D2-D5 — 1N4007 rectifier diode B1 — PB27 ion buzzer DIS1,DIS2 — LTS542 common-anode
display
LED2 — D.P. of DIS-1 display
Resistors (all 1/4 watt, ±5% carbon unless stated otherwise): Rl — 220-ohm R2 — 6.8-kilohm R3 — 2.7-kilohm R4, R5, R8, R10 — 1-kilohm R6, R7 — 10-kilohm R9 — 270-ohm R11 to R24 — 330-ohm R25, R26 — 680-ohm VR1 — 4.7-kilohm preset VR2 — 220-kilohm preset Capacitors: C1,C5,C6,C8 — 0.1μF ceramic disc C2 — 0.01μF ceramic disc C3 — l0μF, 16V electrolytic C4 — l000μF, 25V electrolytic C7 — l00μF, 16V electrolytic Miscellaneous: X1 — 230V AC primary to 12V, 500mA sec. transformer F1 — Fuse with holder PCB-1 — PCB for components PCB-2 — PCB for display S1 — SPDT switch S2 — Push-to-on switch S3 — SPST switch J1,J2 — Jumper wires
— IC sockets, DIL (two 8-pin, two 14-pin and two 16-pin) — LED holder
— Heatsink for IC7 (TO-220 package)
— Convex lens and plane glass
— Nut bolts and screws — Wooden cabinets (three)
Fig. 10: Back view of the cabinet. Fig. 9: Front panel layout.
duce mains voltage to 12 volts, diodes D2 through D5 for converting AC volt- age to DC, capacitors C4 through C7 for filtering the DC and 3-terminal posi- tive voltage regulator (IC7) for a con- stant voltage. C5, C6 and C8 also act as surge capacitors. LED4 monitors the working of the circuit.
Since TTL ICs require a regulated power supply, it should be ensured that
the regulated voltage remains within 4.5 to 5.5 volts.
Installation
To simplify installation, the unit may be split up into three parts: (a) source of light, (b) phototransistor, and (c) the rest, comprising detector, counter, timer with alarm, display and
power supply.
The IR-LED and phototransistor can be fitted on either side of the con- veyor belt or the entrance gate (about 90 cms above ground level). Convex lens may be used to focus the IR light beam on phototransistor. The box, con- taining the rest of the circuit, may be
kept elsewhere.
Readers’ Comments:
The author may please clarify some of my doubts:
Can a 6V stepdown transformer be used, instead of a 12V transformer?
How could this project be used as a burglar alarm and interruption counter together? Can it be used by connecting the points B and C together with point A?
What should be the maximum dis- tance between the IR transmitter and the receiver?
Could the buzzer be replaced with a speaker?
Sandeep Mathur Delhi In the counter there are two ICs used to drive one display and a total of 4 ICs for the two displays. This will make the PCB more complicated which will thus cost more. Instead of using ICs 74LS247 (BCD to decimal decoder) and a 7-segment display for counter 1, only a CD4033 can be used for one display. It has both decade counter as well as BCD to decimal decoder and in-built circuit, limiting resistors and drives a common-cathode 7-segment display, thus reducing the size and cost of the PCB.
R. Sasidhara Reddy Bangalore
The author, Shamsundar Chendake, replies:
As explained in my article, TTL ICs require regulated +5V supply. If voltage across TTL ICs increases be- yond 5.5 volts the ICs will get dam- aged, and if voltage falls below 4 volts the ICs will not give proper performance. Thus, this project needs regulated voltage and the input volt- age (through the regulator) must be 3 volts higher than the output volt- age.
This circuit can be used both as burglar alarm as well as interruption
counter.
The maximum distance between the IR transmitter and the receiver should be 2.4 metres, but if you install another convex lens at IR diode (LED1) the distance can be increased up to 7.6 metres.
The buzzer may be replaced after this modification as shown.
Here IC 555 is used as a stable multivibrator which produces 1kHz au- dio tone at its output.
IC CD4033 (or CD4026) can be used instead of two ICs (7490 and 74247).
SECTION
B:
output is ‘low’. The sum odd output is the complement of the sum even out- put.
The input signals are applied by closing switches S1 to S9, which is indicated and confirmed by red light of the bicolour LEDs. Absence of logic high input signals is ensured by opening of switches S1 to S9, indi- cated and confirmed by green light of the bicolour LEDs.
N1 to N9 are the inverting gates of
7404 ICs to provide alternate light of bicolour LED. IC1 accepts nine inputs. Its outputs are obtained at the fifth pin as even and at the sixth pin as odd. Diodes D1 and D2 and display LTS-543 are used to show E (even) and O (odd), according to the sum of input signals. Red light of bicolour LED is considered for total logic high inputs to decide parity signal at the output. This circuit works well off 4.5V DC supply for any combination of input signals.