Two Transistors Audio Amplifier

ping pong circuit schematic

Rain Detector

This circuit uses a sensor made of a small piece of etched PC board and a simple SCR circuit to detect rain and sound a buzzer. The SCR could also be used to activate a relay, turn on a lamp, or send a signal to a security system.

Parts



R1(1)1K 1/4 W Resistor
R2(1)680 Ohm 1/4 W Resistor
D1(1)1N4001 Silicon Diode
BZ1(1)12V Buzzer
S1(1)SPST Switch
SCR1(1)C106B1 SCR 106CY
SENSOR(1)See Notes
MISC(1)Board, Wire, Case, PC Board (For Sensor)


Notes

1. The sensor is a small piece of PC board etched to the pattern showen in the schematic. The traces should be very close to each other, but never touching. A large spiral pattern would also work.


2. Make sure to use a loud buzzer.

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Mini-MosFet Audio Amplifier

Notes
This project was a sort of challenge: designing an audio amplifier capable of delivering a decent output power with a minimum parts count, without sacrificing quality.
The Power Amplifier section employs only three transistors and a handful of resistors and capacitors in a shunt feedback configuration but can deliver more than 18W into 8 Ohm with <0.08% THD @ 1KHz at the onset of clipping (0.04% @ 1W - 1KHz and 0.02% @ 1W - 10KHz) and up to 30W into a 4 Ohm load.
To obtain such a performance and to ensure overall stability of this very simple circuitry, a suitable regulated dc power supply is mandatory. This is not a snag because it also helps in keeping noise and hum of the preamp to very low levels and guarantees a predictable output power into different load impedances. Finally, as the amplifier requires only a single rail supply, a very good dc voltage regulator capable of supplying more than 2 Amps @ 40V can be implemented with a few parts also.

Interior light extender

1. Mount components to the PCB from the PLAIN SIDE.  Polarity must be observed for DI, (Power Diode) and for CI (Electrolytic Capacitor).
(i) DI - Band on Diode is at K (-ve) end
(ii) CI - Arrow on Body of Capacitor points down the negative leg.
Push the component legs through, nip off 1-2mm clear of the bus-bars and bend leg's in opposite directions. This will hold the components in place.
2. TI (BC558) should be mounted with the flat section in the direction shown.
3. T2 can be mounted as follows:
(1)  Turn the end of a piece of link wire to a ring and solder together. This ring is attached to a hole in the transistor body by a 1/2 x 1/8" Metal thread screw. The link wire is cut and soldered at B1 - B6.
(2) The legs on the transistor are too large to fit into the holes in the Veroboard so solder short lengths of link wire to the transistor legs and solder these to the PCB.
4. Solder electrical wire from Kit to 12V + and 12V - positions.
5. To wire to automobile circuit, the following procedure is recommended.
(a) Locate the wire from the Interior Bulb to the switch activiated by the car door.
(b) Determine that the car has a Positive Earth or Negative Earth Wiring system by checking the battery. (Most cars use negative earth).
(c) For Negative Earth System join the 12V +ve wire from your project to the wire from bulb to switch using the wire-tap from the Kit. For Positive Earth system join 12V -ve wire instead.

(d) Ensure that the cover on the wire-tap is folded into place so that no bare metal is exposed.
(e) Join the 12V -ve wire from your project to a suitable earth point on the car body. (For Positive Earth system read 12V +ve).
NOTE: Do not cut existing wiring to the interior light just join the project in parallel with the switch.
(f) Test the unit for correct function.
TROUBLE SHOOTING
(i)  Check for dry joints, joints not soldered at all, or solder bridging PCB tracks.
(ii)  Check polarity of D1 (Diode) C1 (Electrolytic Capacitor) T1 BC558 PNP Transistor and T2 2N3055 NPN Transistor must be mounted as shown in diagram.

(iii) Check locations of all components.

Technology Notes

All modern cars are fitted with door-switch operated courtesy lights. Useful devices, but not quite as useful as they might be because they are so arranged that the light is extinguished as soon as you close the door - just when you need light to find the ignition switch. do up your seat belt etc. How much better if the internal light stayed on for a few seconds after the door is closed.  This little project does just that. It provides a four-second delay (approx) after which the interior light slowly dims - being finally extinguished after 10 or 12 seconds.  The unit is very simple to construct and once tested and properly insulated it may be wired across one of the car door switches.  In operation, after a short delay the lights will gradually dim until they are completely extinguished. There is no battery drain in the off-state as the unit only operates during the delay period after the door is closed.

Most car door switches am simply simglec-pole switches. with one side earthed.  When the door is opened the switch earths the other line thus completing the Light circuit.

In a car whem the negative terminal of the battery is connected to the chassis the nerative wire of the unit (emitter of Q2) is connected to the chassis and the positive wire (case of 2N3055) Is connected to the wire going to the switch.  In. a car having a positive earth system this connection sequence is reversed.

When the switch closwe (door open) C1 is discharged via DI to zero rolts and when the switch opens, C1 charges up via R1 and R2. Transistors QI and Q2 are connected as an emitter follower (Q2 just buffers Q1) therefore the yoltage across Q2 increases slowly as C1 charges.  Hence Q2 acts Like a low resistance in pars1lel with the switch - keeping the lights on.

The value of C1 is chosen such that a useful light level is obtained for about four seconds, thereafter the light decreases until in about 10 seconds it is out completely.  With different transistor gains and with variation to current drain due to a particular type of car the timing may be simply adjusted by selecting C1.

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15W mono car amplifier using TA7227P

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Build a good Audio Buffer Amplifier

General Description:
This circuit will amplify voice audio in the range of 50 to 10,000 cycles with little distortion, and have the ability to drive a low impedance load to 16 ohms.  The circuit will run from 6 to 15 vdc and give about 20 dB of gain.
Concept:
To build an inexpensive audio amplifier with little parts count that is very reliable and easy to build and implement. Parts are available from Radio Shack.
Construction:
Build the circuit on a breadboard and use a socket so when the lightning hits, well you know......
If the audio input is biased above ground (audio floats on a voltage) a coupling cap will need installed in the "Audio In" lead to the pot.  The pot can be any value from 5 K to 100 K.  The 100 pF helps eliminate RF bombardment, making it suitable for higher RF environments. Don't forget the 100 uF cap or the circuit will oscillate.
Many op-amp circuits don't drive low impedance loads well, this circuit will handle a load impedance to about 16 ohms but doesn't need to be loaded down to that impedance.  The output cap is a 10 uF non polarized electrolytic for impedance's to 600 ohms.  The output capacitance should be raised to 100 uF for impedance's to 100 ohms and to 1000 uf for impedance's below 100 ohms (speaker.)
Operating bandwidth is from 50 cycles to 10 kc.  Over 10 kc and the unit suffers from poor slew rate, causing distortion, but for NBFM this bandwidth is acceptable.  Actually, for this purpose slew rate limitations work to our advantage as it helps make the amplifier less RF susceptible.  Increased audio amplification can also be had with the addition of a feedback loop.  Consult the RS parts substitution manual for examples.

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