The following circuit diagrams are tone detector circuit diagrams which also known as sound activated switch circuit. Actually, these circuits  use microntroller for switching because the circuits were designed for robot start up activation. But you can build switching module using relay.

Tone Detector diagram 1

Tone Detector diagram 2

Tone Detector diagram 3

Tone Detector diagram 4

Tone Detector diagram 5

Source: Sound activated switch for robot

This car headlight alarm circuit can be set for one or two functions:
First, to indicate that the head lights (or the side lights) should be switched off after switching off the ignition contact. With this circuit, there should be no dead battery due to headlights that were left on.
Second, to indicate that the head lights should be on once ignition contact is switched on.

Car Headlight Alarm Features:

• Continuously repeated alarm tone for lights ON (may be disabled)
• Repeated alarm tone for lights OUT
• Only 3 wires are required for hook-up

Download the manual of Car Headlight Alarm here (you may buy the kit at electronickits.com with price of US$19.95)

This transistor ignition circuit give your car to have better starting and smoother running, particularly at very high and very low RPM. Lower fuel consumption, less pollution, lower servicing costs. Drive economically, drive electronically. Only for petrol/gasoline engines. This circuit will reduce breaker point wear and provide cleaner spark.

Circuit diagram:

Wiring diagram with the vehicles:

Notes:
This circuit used for cars with negative ground 12V negative ground system, maximum ignition current of 4A and maximum switching speed 500KHz. Motorcycles, mowers, boats, etc can also use this circuit.

For 6V Negative ground system, change the following resistors:

• R1, R2: 150 ohm / 1W
• R3 : 68 ohm / 1/4W
• R4 : 100 ohm / 1/4W
• R5, R6, R7: 68 ohm / 1W

The kit of Electronic Transistor Ignition is available at electronickits.com, sell at $25.95.
Download the manual Transistor Ignition kit HERE

This is the circuit diagram of a voice operated relay. It similar with sound activation switch circuit which will turn on and turn off (connect and disconnect) the switch depending on the sound input. The output switch of this circuit is act by a relay.

Component Parts list:

R1, R7 1K
R2, R4, R8 10K
R3 2K2
R5 1M
R6 200K
R9 39K
D1, D4 1N4004
D2, D3 1N4148
C2, C5 2.2uF/16V mini ecap

C1 10uF/16V mini ecap
C4 100pF
C3 100nF
Q1 BC548
IC1 LM358 (of Hitachi HA17358)
8 pin IC socket
Relay AX-SH-112L
LED 3mm
Electret microphone

This mini-VOX – voice operated relay – is based on a circuit published in Silicon Chip, 9/1994, p31.

The off delay time may be adjusted by varying R3 and R4. Reducing R3 will result in a longer release time.

You could change the release time constant (C2 & R5) to say 30 seconds and use the VOX as a light switch with this delay time before turning off. Increase C2 to say 10uF and R3 to 3M3.

To make the easy adjustment, you may use potensiometer or trimpot to varying the value of resistor. You will need additional place on the PCB but it will easier for you to make some adjustment

The kit of this circuit available at kitsrus.com anf electronickits.com, you may buy the circuit there.
HERE the manual instruction of this Mini Voice Operated Relay circuit

This is the circuit diagram of Line Follower / Line Tracker robot. The circuit taken from the tutorial documentation. You may download the full tutorial at the end of this article.

The line follower robot use 8 pieces of proximity sensor module. The sensor module use photodioda for detecting the reclection of light from the line/floor.
Proximity / Line Detector circuit:

ATmega8535 is a cheap and great microcontroller for your robot. This microcontroller is very easy to use, very easy to find, very easy to program.
Microcontroller ATmega8535 circuit:

The motor driver L298 able to control the motor with current output up to 2A for single DC motor. For double DC motors, there should be up to 1A for each output channel. This motor driver is very good for small and medium DC motor. L298 is great motor driver for small and medium size or robot such as line follower robot and fire fighter robot.
Motor Driver circuit with L298:

Download the complete line follower robot turotial which containing od schematic diagram, robot design and source code in PDF version (Indonesian language):
Download Link

Here the complete electrical circuit diagram of line follower robot which built based on ATmega16. There are three modules of line follower robot circuit that are sensor module, microcontroller module and DC motor module.

IR sensor schematic diagram:

Mainboard (microcontroller + DC motor driver schematic diagram):

Download the document of ATmega16 line follower robot tutorial for complete tutorial including the working explanation of circuit and program, source code and schematic diagram.

This circuit is a wireless car alarm system that is built using two circuit modules, namely modules of transmitter and receiver modules. This circuit works on FM radio waves. Car alarms can be used on vehicles that have a 6-12VDC power supply. You can use the voltage stabilizer if your car power supply is too large (eg 24V).

How the circuit works:
The mini VHF, FM transmitter is fitted in the vehicle at night when the car is parked in the car porch. The receiver unit is built with CXA1019. CXA1019 a single IC-based FM radio module which is freely available in the market, is kept inside.

Receiver is tuned to the transmitter’s frequency. When the transmitter is on and the signals are being received by FM radio receiver, no hissing noise is available at the output of receiver. Thus transistor T2 (BC548) does not conduct. This results in the relay driver transistor T3 getting its forward base bias via 10k resistor R5 and the relay gets energised.

When an intruder tries to drive the car and takes it a few metres away from the car porch, the radio link between the car (transmitter) and alarm (receiver) is broken. As a result FM radio module will generate hissing noise. Hissing AC signals are coupled to relay switching circuit via audio transformer. These AC signals are rectified and filtered by diode D1 and capacitor C8, and the resulting positive DC voltage provides a forward bias to transistor T2. Thus transistor T2 conducts, and it pulls the base of relay driver transistor T3 to ground level. The relay thus gets de-activated and the alarm connected via N/C contacts of relay is switched on.

If the intruder finds out about the wireless alarm (which already turned on) and disconnects the transmitter from battery, still remote alarm remains activated. It will not turn off the alarm because in the absence of signal, the receiver continues to produce hissing noise at its output. So the burglar alarm is fool-proof and highly reliable.

Circuit diagram source: www.electronic-circuits-diagrams.com

This is a car alarm simulator which using the LED as a simulation output. This simple circuit can tell you whether your car is running or not by detecting the voltage difference when the car is on and off. This occurs because when your car is running the Alternator puts a out a voltage a little bit higher than when the car is off.

The circuit will be activated automatically when the engine is turned off.

How to run the circuit:
1. Connect the circuit to the car electrical system.
2. Adjust RV1 until the LED flashes when the engine is not running
3. Start the engine, the LED should turn off. If the LED still on, the andjust the RV1 slighty

This circuit actually is a kits, you can get the kits at http://www.electronickits.com/kit/complete/surv/vemk126.htm. But it is very possible to you to build your own car alarm simulator circuit.

Download the circuit manual HERE.

This is an 250mW RF power amplifier project. This circuit is designed to amplify the output of about 7mW wide band FM transmitters to a final level of about 250mW.

Schematic Diagram:

Component Parts:

R1 ____________________ 10R
R2 ____________________ 470R
R3 ____________________ 47R
R4 ____________________ 120R
R5 ____________________ 2K2
R6 ____________________ 4K7
R7 ____________________ 22R
Q1 ____________________ 2N2369
Q2 ____________________ 2N4427
Q3 ____________________ 78L08

C1, C11 _______________ 10n
C2 ____________________ 220u/16V
C3 ____________________ 33p
C4 ____________________ 47p
C5, C6, C7, C8, C9 ____ 1n
C10 ___________________ 10u/25V
L1 ____________________ 5 turn coil
L2 ____________________ 3 turn coil
L3 ____________________ Ferrite
L4, L5, L6 ____________ RFC

This electronic circuit is a simple 2-transistor VHF power amplifier, with about 16dB gain, and requires no tuning or alignment procedures. Wideband techniques have been used in the design and the circuit is equipped with a “lowpass” filter to ensure good output spectral purity. The project has been designed for assembly on a single-sided printed circuit board. The circuit is specifically designed to amplify the output of 7mW to 10mW WBFM transmitters (wide band) to a final level of 250mW to 300mW, after the filter.

The kit of this power amplifier circuit is available at kitsrus.com. You may download the kit manual including the information of schematic diagram, component parts, and circuit works HERE

This simple 220V light dimmer circuit can be used to adjust the brightness of mains lights. It also can be used to adjust the speed of AC motors. It uses a triac, diac and has a radio-frequency interference (RFI) noise suppression circuit built into it as well.

Schematic diagram:

Component part:
R1 = 10K
R2 = 2K2
R3 = 180K
Potentiometer 250K
C1 = 150nF/400V dipped polyester
C2 = 47nF/630V  dipped polyester
C3 = 220nF/250V AC dipped polyester
Triac = 2N6075 or BT136-500D
Diac = HT-32
FUSE 2A or 3A

Download the circuit documentation of 220V/240V Light Dimmer. The kit of this circuit is available at kitsrus.com, please visit the website to purchase the kit.