S.C. DwiveDi
T
his LED-based message display is built around readily availble, low- cost components. It is easy tofab-ricate and makes use of 3mm red LEDs.
A total of 172 LEDs have been arranged to display the message “HAppY NEW
YEAR 2004.”
The arrangement of LED1 through LED11 is used to display ‘H’ as shown
in Fig. 1. The anodes of LED1 through LED11 are connected to point A and the cathodes of these LEDs are connected to point B. Similarly, letter ‘A’ is built using LED12 through LED21. All the anodes of LED12 through LED21 are connected to point A, while the cathodes of these LEDs are connected to resistor R8 (not shown in the circuit diagram). Other letters/words can also be easily arranged to make the required sentence.
The power supply for the message display circuit (Fig. 2) comprises a 0-9V, 2A step-down transformer (X1), bridge rectifier comprising diodes D1 through D4, and a filter capacitor (C1). IC 7806 (IC1) provides regulated 6V DC to the display circuit comprising timer 555 (IC2) and decade counter CD4017 (IC3). The astable multivibrator built around IC2 produces 1Hz clock at its output pin 3. This output is connected to clock pin (pin 14) of the decade counter.
The decade counter can count up to 10. The output of IC3 advances by one count every second (depending on the time period of astable multivibrator IC2).
When Q1 output of IC3 goes high, transistor T1 conducts and the current flows through LED1 through LED48 via resistors R7 through R11. Now the word
‘HAppY’ built around LED1 through LED48 is displayed on the LED arrange-ment board.
Next, when Q2 output of IC3 goes high, transistor T2 conducts and the cur-rent flows through LED49 through LED87 via resistors R12 through R14. Now the word ‘NEW’ is displayed on the LED ar-rangement board.
Again, when Q3 output goes high, transistor T3 conducts and the current Fig. 1: LED arrangement for word ‘H’
Fig. 2: Circuit diagram of LED-based message display
flows through LED88 through LED128 via resistors R15 through R18. Now the word ‘YEAR’ is displayed on the LED ar-rangement board.
Similarly, when Q4 output goes high, transistor T4 conducts and the current flows through LED129 through LED172 via resistors R19 through R22. Now digits
‘2004’ are displayed on the LED arrange-ment board.
During the entire period when Q5, Q6, Q7, or Q8 output go high, transistor T5 conducts and the cur-rent flows through all the LEDs via diodes D9 through D12 and resistors R7 through R22. Now the complete message “HAppY NEW YEAR 2004” is displayed on the LED arrangement for four seconds.
Thus, the display board displays
‘HAppY,’ ‘NEW,’ YEAR’ and ‘2004’ one after another for one second each. After that, the message “HAppY NEW YEAR 2004” is displayed for 4 seconds (because Q5 through Q8 are connected to resistor R6 via diodes D5 through D8).
At the next clock input output Q9 goes high, and IC3 is reset and the display is turned off for one second. Thereafter the cycle repeats.
dC-to-dC Converter
PrinCe PhilliPS
H
ere’s a low-cost circuit to convert 6V DC into 12V DC. It uses no transformer and is easy to con-struct with few components.The circuit is built around IC 555, which generates the required frequency of around 2 to 10 kHz to drive power transistor BD139 (T2). The output fre-quency of the IC can be adjusted by a 47k potmeter (VR1) and given to the base of transistor T2 via resistor R3. Transistor T2 is mounted on an aluminium heat-sink. Inductor L1 and capacitor C5 (2200µF, 35V) are energy storage components. The 12V zener di-ode regulates the voltage across the output of the circuit.
The inductor comprises 100 turns of 24SWG enamelled copper wire wound on a 40mm dia. toroidal ferrite core. The more the turns on the core, the higher the current delivering capability of the circuit to the load at the output.
The output current is controlled by transistor BC549 (T1) with the help of
resistors R4 and R5. The output volt-age is controlled by the zener diode and smoothed by capacitor C5.
You can obtain regulated 12V DC, 120 mA across the output of this circuit.
At higher loads (below 100 ohms), the
circuit might not perform well and deliver as much current. Use a large capacitor (C5) and inductor for higher voltages and higher currents, respectively. Different output voltages can be obtained by using zener diodes of other ratings.
versAtIle proxImIty deteCtor wIth Auto reset
KauShiK hazariKa
E
lectrochemical processes taking place in our body generate com- plex signals (hum) that arecontinu-ously being passed along the nerve fibres throughout the body. Any physical activ-ity such as muscle movement increases
hum.
Here’s a circuit that operates when it detects hum generated by the human body
in proximity. Its versatility lies in the fact that you don’t need to touch the metal plates for detection. Just the presence of your hand/body within 1 cm of the sensing loop triggers the circuit. The activation of the circuit is indicated by the glowing of an LED and an audible beep. The circuit continues to glow and beep until the hand
is within 5 cm of the loop. Beyond 5 cm, it resets automati-cally.
Here IC2 (555) simplifies the cir-cuitry otherwise needed to achieve this. Regulator 7809 (IC1) supplies 9V DC to the circuit.
When power is turned on, capacitor C3 (47 kpF) charges through resistor R1 (1 mega-ohm). Output pin 3 of IC2 remains high as long as the voltage at its pin 2 is below 2/3Vcc; the buzzer beeps for this period. Beyond that voltage, the output resets (goes low).
Transistors T1 and T2 (each BC548) form a Darlington pair. As long as T1 and T2 remain in cut-off condition, capacitor C3 retains the charge and the buzzer is off. When you take your hand within 1 cm of the loop wire, T1 conducts due to the noise picked up by its base. So capacitor C3 gets a discharge path, and the volt-age at pin 2 of IC2 going below 1/3Vcc sets output pin 3 high. As a result, the buzzer sounds.
The beep continues until C3 charges to 2/3Vcc due to gradual withdrawal of the hand from vicinity of the loop wire. The series combination of capacitor C5 and resistor R3 within dotted lines is optional and reduces hum at the base of T1. The values of C5 and R3 to be used for varying the sensitivity of the circuit are given in the table.
For calibration, wire the circuit and use a 7cm hook-up wire at the base of T1. When you place your hand over the wire insulation, the buzzer should beep.
If it doesn’t, check connections. Now connect the loop wire. If beep continues even when there is no person within 20 cm, use a suitable combination of C5 and R3 from the table to reduce the circuit sensitivity.
The suggested pCB size for the circuit (excluding power supply) is 4 cm×3 cm.
Solder the loop wire directly. A small hook-up wire was used in the prototype.
Do not remove insulation of the wire.
Keep the circuit away from mains wiring and large metal objects.