Simple Crystal Tester

Transistor Q1, a 2N3563, and its associated components form an oscillator circuit that will oscillate if, and only if, a good crystal is connected to the test clips. The output from the oscillator is then rectified by the 1N4148 signal diode and filtered by C3, a 100pF capacitor. The positive voltage developed across the capacitor is applied to the base of Q2, another 2N3563, causing it to conduct. When that happens, current flows through Led1, causing it to glow. Since only a good crystal will oscillate, a glowing LED indicates that the crystal is indeed OK. 

You can use the NTE123AP, PN100, or the 2N3904, for the transistors, however, the circuit works better with the 2N3563's for crystals in the higher MHz. The NTE108 is a direct replacement for the 2N3563. R3, the 330 ohm resistor for the Led, can be lowered to 220/270 ohm if your application is for crystals in the high MHz, which makes the led glow dimly in some instances. 

The circuit is powered by a standard 9-volt battery and the SPST push button power-switch is included to prolong battery life but not really needed if you use a socket for the crystal-under-test. I'm using this particular simle tester myself.

 

Parts List:
R1 = 100K C1,C2 = 220pF
R2 = 2K2 C3 = 100pF
R3 = 330 Ohm C4 = 0.1uF (100 nF)
Q1,Q2 = 2N3563 D2 = red led, ultra-bright
D1 = 1N4148 S1 = Momentary Push-button, SPST (optional)
Miscellaneous: crocodile clips (or others), 9V battery clip, wire, solder, etc

Source: http://www.electronics-lab.com/projects/test/010/index.html

Electronic Stethoscope

Circuit Description:

U1a operates as a low-noise microphone preamp. Its gain is only about 3.9 because the high output impedance of the drain of the FET inside the electret microphone causes U1as effective input resistor to be about 12.2K. C2 has a fairly high value in order to pass very low frequency (about 20 to 30Hz) heartbeat sounds.
U1b operates as a low-noise Sallen and Key, Butterworth low-pass-filter with a cutoff frequency of about 103Hz. R7 and R8 provide a gain of about 1.6 and allow the use of equal values for C3 and C4 but still producing a sharp Butterworth response. The rolloff rate is 12dB/octave. C3 and C4 can be reduced to 4.7nF to increase the cutoff frequency to 1KHz to hear respiratory or mechanical (automobile engine) sounds.
The U4 circuit is optional and has a gain of 71 to drive the bi-colour LED.
U5 is a 1/4W power amplifier IC with built-in biasing and inputs that are referred to ground. It has a gain of 20. It can drive any type of headphones including low impedance (8 ohms) ones.

R1	1	10K 1/4W Resistor
R2	1	2.2K 1/4W Resistor
R3, R9	0	Not used
R4	1	47K 1/4W Resistor
R5, R6, R7	3	33K 1/4W Resistor
R8	1	56K 1/4W Resistor
R10	1	4.7K 1/4W Resistor
R11	1	2.2K to 10K audio-taper (logarithmic) volume control
R12	1	330K 1/4W Resistor
R13, R15, R16	3	1K 1/4W Resistor
R14	1	3.9 Ohm 1/4W Resistor
C1, C8	2	470uF/16V Electrolytic Capacitor
C2	1	4.7uF/16V Electrolytic Capacitor
C3, C4	3	0.047uF/50V Metalized plastic-film Capacitor
C5	1	0.1uF/50V Ceramic disc Capacitor
C6, C7	2	1000uF/16V Electrolytic Capacitor
U1 U2, U3 U4 U5	1 0 1 1	TL072 Low-noise, dual opamp Not used 741 opamp LM386 1/4W power amp
MIC	1	Two-wire Electret Microphone
J1	1	1/8" Stereo Headphones Jack
LED	1	Red/green 2-wire LED
Batt1, Batt2	2	9V Alkaline Battery
SW	1	2-pole, single throw Power Switch
Misc.	1	Stethoscope head or jar lid, Rubber Sleeve for microphone.

Assembly:

1) Assemble the circuit using Veroboard (stripboard) or a PCB.
2) Use a shielded cable for the microphone as shown on the schematic.
3) Fasten the microphone to the stethoscope head with a rubber isolating sleeve or use a short piece of rubber tubing on its nipple. A thick jar lid can be used as a stethoscope head. The microphone must be spaced away from the skin but the stethoscope head must be pressed to the skin, sealing the microphone from background noises and avoiding acoustical feedback with your headphones.
4) The microphone/stethoscope head must not be moved while listening to heartbeats to avoid friction noises.
5) Protect your hearing. Keep the microphone away from your headphones to avoid acoustical feedback.

Source: http://www.electronics-lab.com/projects/science/003/

TDA2005 - Low Cost 2x20 Watt Stereo Amplifier

This circuit is a small stereo amplifier for all suitable applications like amplifying small speakers, boxing, etc. It is also suitable for car use but before, the power supply must be choked with at least 150mH and it must give up to approximately 6 to 7 amps during the upstream performance. Appropriate heatsink for the amplifier is SK08 with a height of 50 mm (approx. 2.5 K per watt). You should drill the cooler after soldering the board to center it properly. The TDA2005 also needs not be isolated from the heat sink, since the metal mounting part of the IC is grounded.

You should use thermal paste to improve the heat dissipation. After the assembly , case construction is left to the builder. 100K potentiometers are used for adjusting the input volume. The potentiometers are absent in the layout. The 100K resistors need only be installed if the 100 K potentiometers are not used as shown in the layout. You should use a well designed quality transformer to get less noise. It will be another good way to use a sufficient battery to power the circuit. Keep the supply wires as short as possible. Input source should be isolated from the external noises too. It is recommended to use coaxial cable to connect the input audio.

Technical data:Performance of TDA2005M: (for this circuit); At 14.4 V supply voltage: 2 x 20 watts (stereo) into 4 Ohms.
Distortion: Approx. 0.2% at 4 Watts into 4 ohm load.
Frequency Range: Approx. 20 Hz to 22 KHz.
Input Sensitivity: Approx. maximum 150 mV rms. .
Power supply: + 8 to 18 volts, approx. maximum 3.5 Amps per channel.
Click Here to Download Schematic, PCB and Layout Files

Infra-Red Remote Control Tester

This little circuit is invaluable for quick go/no-go testing of just about any remote control transmitting infra-red (IR) light. The tester is battery-powered, built from just a handful of commonly available and inexpensive parts, and fits in a compact enclosure. Schmitt trigger gate IC1f is used as a quasi-analogue amplifier with, unusually, an infra-red emitting diode (IRED) type LD274 acting as the sensor element. An R-C network, C1-R2, is used at the output of the gate because all IR remote controls transmit pulse bursts, and to prevent the output LED, D2, lighting constantly when day-light or another continuous source of IR light is detected.

Circuit diagram:



Cased project:

This creates a useful ‘quick test’ option: point the tester at direct daylight, and the indicator LED should light briefly. The sensitivity of the tester is such that IR light from remote control is detected at a distance of up to 50 cm. The circuit is designed for very low power consumption, drawing less than 1 mA from the battery when IR light is detected, and practically no current when no light is detected. Hence no on/off switch is required. The construction drawing shows how the tester may be ‘cased’ using a small ABS case from Conrad.

COMPONENTS LIST
Resistors:
R1,R2 = 10MW
Capacitor:
C1 = 10nF
Semiconductors:
D1 = LD274 (Siemens)
D2 = LED, 3mm, low-current
IC1 = 74HC14
Miscellaneous:
Bt1 = 3V Lithium cell with solder tags, e.g.type CR2045 (560 mAh)

source: http://www.extremecircuits.net/

Contactless AC Mains Voltage Detector

This is a CMOS IC (CD4033) based circuit which can be used to detect presence of AC mains voltage without any electrical contact with the conductor carrying AC current/voltage. Thus it can be used to detect mains AC voltage without removing the insulation from the conductor. Just take it in the vicinity of the conductor and it would detect presence of AC voltage. If AC voltage is not present, the display would randomly show any digit (0 through 9) permanently. If mains supply is available in the conductor, the electric field would be induced into the sensing probe. Since IC used is CMOS type, its input impedance is extremely high and thus the induced voltage is sufficient to clock the counter IC. Thus display count advances rapidly from 0 to 9 and then repeats itself. This is the indication for presence of mains supply. Display stops advancing when the unit is taken away from the mains carrying conductor. For compactness, a 9-volt PP3 battery may be used for supply to the gadget.

source: http://www.extremecircuits.net/