Manual Reference

Although this design is reproduced directly from the manufacturer’s datasheets, its use in this application is rather novel. Originally intended for high-visibility LED bargraph readouts, here the LM3914 is used as the basis of a 10-step variable brightness current-regulated white LED torch! The circuit has only four components in the control and regulation circuit: R1, R2, VR1 and the LM3914. The circuit can be built directly on the pins of the LM3914 to produce a package not much bigger than the LM3914 itself. The LM3914 is set to operate in bargraph mode so that the LEDs light progressively as its input signal increases.

This signal comes from the wiper of VR1, which provides a variable voltage between 0V and the supply voltage to pin 5 of the LM3914. The internal resistor ladder network of the LM3914 has its low end (pin 4) connected to ground and the high end (pin 6) connected to the supply voltage via R2. The purpose of R2 is to give LED 10 a clear turn-on zone. Resistor R1 (620Ω) on pin 7 of IC1 sets the current through each LED to about 20mA. As VR1 is rotated from the 0V position (all LEDs off) to the supply voltage position (all LEDs on), the LEDs will progressively light. With all LEDs off, the circuit will draw about 5mA. With all LEDs illuminated, it will draw about 205mA and dissipate 307mW with a 4.5V supply.

Editors note:

These are nominal figures only. Actual device dissipation will depend entirely on the input voltage and LED forward voltage. In use, we recommend that a resistor (R3) be inserted in series with the positive supply, chosen so that the LM3914’s dissipation is limited to about 500mW. Typically, this would be needed for supply voltages of 6V and higher. The inclusion of the resistor necessitates a 10μF decoupling capacitor across the supply rails.) By carefully selecting the LEDs, this torch can be as bright as 15,0000mCd while costing less than $20.

Here’s a twist to the age-old game of noughts and crosses. Instead of pen and paper, it uses nine 10mm tri-colour LEDs arranged in a 3 x 3 grid. One player has nine red buttons while the other player has nine green, set out in identical grids. The aim, of course, is to make three LEDs in a row glow the same colour – red or green! Pushing a red button causes the LED in the equivalent position on the grid to glow red in colour. Likewise, pushing a green button lights the equivalent green LED.


If a player pushes a button for a LED that is already glowing red or green, then that LED changes to yellow, exposing the false move! All the LEDs are then turned off, ready for the next game, by pressing the "Clear" button. For simplicity, the circuit shows only one tri-colour LED and a pair of opposing buttons. This circuit fragment must be repeated another eight times to create a complete 3 x 3 grid. A brief press on a button fires the associated SCR and turns on the LED. The common (cathode) lead of all LEDs is connected to the 0V rail via the normally-closed contacts of the "Clear" pushbutton (S3).

We have already published designs that use a transistor junction operating in Zener breakdown as a noise source. Anyone who has experimented with a reverse-biased transistor knows that the amplitude of the noise voltage generated in this manner is strongly dependent on the supply voltage. The variation between individual transistors is also rather large. An obvious solution is to use an adjustable supply voltage for the noise generator stage. A BC547B starts to break down at around 8V.

Circuit diagram :

Optimised Semiconductor Noise Source Circuit Diagram

Using P1 and R1, you can adjust the voltage across T1 and R2 between 8 and 12V. C3 decouples the reduced supply voltage. An impedance buffer in the form of T2 and R3 is added to the circuit, to prevent the connected load from affecting the noise source. This buffer is powered directly from the 12-V supply. To adjust this circuit, connect the output to an oscilloscope. Then adjust P1 to obtain the highest signal amplitude, combined with the best ‘shape’ of the noise signal. The output voltage is approximately 300mVpp, and the current consumption is around 2mA.

Source : www.extremecircuits.net

A number of people have been unable to find the transformer needed for the Black Light project, so I looked around to see if I could find a fluorescent lamp driver that does not require any special components. I finally found one in Electronics Now. Here it is. It uses a normal 120 to 6V stepdown transformer in reverse to step 12V to about 350V to drive a lamp without the need to warm the filaments.

Parts:

C1 100uf 25V Electrolytic Capacitor
C2,C3 0.01uf 25V Ceramic Disc Capacitor
C4 0.01uf 1KV Ceramic Disc Capacitor
R1 1K 1/4W Resistor
R2 2.7K 1/4W Resistor
Q1 IRF510 MOSFET
U1 TLC555 Timer IC
T1 6V 300mA Transformer
LAMP 4W Fluorescent Lamp
MISC Board, Wire, Heatsink For Q1

Notes:

1. Q1 must be installed on a heat sink.
2. A 240V to 10V transformer will work better then the one in the parts list. The problem is that they are hard to find.
3. This circuit can give a nasty (but not too dangerous) shock. Be careful around the output leads.