SOLID STATE TESLA COIL WITH 555 TIMER

Single transistor flyback driver caused many problems due to it's operating principle. In addition, since it's resonance frequency is determined by every part of the system, when you try to draw an arc from the transformer, it changes dramatically in most of the cases. Just because the operating frequency is important for the safety criteria, (both for mine and power transistor's), I decided to make it run on a constant frequency and built up another simple circuit, trying to stay in the specified limits of the 555 timer.

Setting the operating frequency with an integrated timer is easy and practical. Following schematic is nothing more than the standard astable mode circuit design with a classic 555. It requires only two resistors and a capacitor to set frequency (with duty cycle of course) and another resistor to determine power transistor's base current, which you can find it's optimal value experimentally. I used 1K for R1, 2.2K for R2, and 10nF for C which made circuit to run nearly at 27 kHz theoretically, at %60 high to %40 low duty cycle.

Power transistor is not critical and any other may be used as long as it's characteristics are equivalent or better. Here are the technical datas for BD243C for comparison:

Bipolar NPN transistor : BD243C
Casing : TO220
Max. collector current : 6 Amperes
Max. total power : 65 Watts, while case is at 25 degrees Celsius
Transition frequency : 3 MHz
hFE (current gain) : 30 at 300mA (minimum value)

Water Level Indicator Using 7 Segment Display

This water-level indicator uses
a 7-segment display, instead
of LEDs, to indicate the water
level (low, half and full) in the tank.
Moreover, a buzzer is used to alert you
of water overflowing from the tank.
The circuit shows the water level by
displaying L, H and F for low, half and
full, respectively.

Solar Bug

Hide this solar-powered circuit
suitably and see the reaction
of your friends to the chirpy
sound produced by it every few minutes.
In all probability, it will coax
them to find out where the sound is
coming from.
The circuit runs off a miniature solar
power panel, which can be taken
out from an old calculator such as Citizen
CT-500. A panel giving 1.5V to
2.5V is required. Note that the circuit
can work properly from a panel as
small as 3 cm2.
If a digital voltmeter is connected
across capacitor C2, a slow build-up
of voltage can be observed when the
panel is exposed to light. Transistors
T1 and T2 form a relaxation oscillator.
When C1 charges to 0.6V, transistor
T1 conducts and the charge built up
in C2 is discharged through the
piezobuzzer to produce a short beep.
While testing the circuit, the value
of resistor R1 can be reduced to, say, 1
kilo-ohm. Use a good-quality buzzer
to ensure that the sound produced is
loud enough.

CLAP SWITCH

This circuit toggles the LEDs each time it detects a clap or tap or short whistle.
The second 10u is charged via the 5k6 and 33k and when a sound is detected, the negative excursion of the waveform takes the positive end of the 10u towards the 0v rail. The negative end of the 10u will actually go below 0v and this will pull the two 1N4148 diodes so the anode ends will have near to zero volts on them.
As the voltage drops, the transistor in the bi-stable circuit that is turned on, will have 0.6v on the base while the transistor that is turned off, will have zero volts on the base. As the anodes of the two signal diode are brought lower, the transistor that is turned on, will begin to turn off and the other transistor will begin to turn on via its 100u and 47k. As it begins to turn on, the transistor that was originally turned on will get less "turn-on" from its 100u and 47k and thus the two switch over very quickly. The collector of the third transistor can be taken to a buffer transistor to operate a relay or other device.

SIMPLE MOTOR SPEED CONTROL

This circuit is better than reducing the RPM of a motor via a resistor. Firstly it is more efficient. And secondly it gives the motor a set of pulses and this allows it to start at low RPM.It's a simple Pulse-Width circuit or Pulse-Circuit.

Simple Transistor Circuit

Connect the LED, 220 ohm resistor and transistor as shown in the photo.
Touch the top point with two fingers of one hand and the lower point with
fingers of the other hand and squeeze.
The LED will turn on brighter when you squeeze harder.

Your body has resistance and when a voltage is present, current will flow though your body (fingers). The transistor is amplifying the current through your fingers about 200 times and this is enough to illuminate the LED.