Wireless Doorbell Transmitter Circuit

Posted by Micro System

on Thursday, August 5, 2010

This is a circuit for transmitter that can be used in doorbell circuit. This circuit is a simple form. The circuit is based on three transistor. This is the figure of the circuit;


The 303MHz oscillator consists of a self-oscillating circuit made up of the coil on the PC board and a 9p (9 puff) capacitor (actually 4p and 5p in parallel). The circuit starts-up by the transistor producing noise. This rising-and-falling signal on the collector is passed to the parallel tuned circuit (the tank circuit) and the base sees a very smooth sine wave at a frequency of 303MHz. This sine wave is then amplified by the transistor and this is how the 303MHz frequency is generated. Now we come to the purpose of the 15microhenry choke on the tank circuit. When the circuit oscillates, it takes a larger and small amount of current. This current passes through the choke and the turns produce a back-emf or back voltage that fights against the flow (change) in current. The end effect is a voltage created at the point where the choke is connected to the track-work on the board. This effectively allows the track work to produce a waveform and since the frequency of this wave is very high, a percentage of the energy is radiated into the air as electromagnetic energy. The choke allows the track-work to effectively rise and fall while providing a very low resistance path for the flow of current during certain parts of the cycle.

Door Knock Beeper Circuit

Posted by Micro System

on

This is a design circuit for alarm circuit that can be provides a means to activate a loud beeping sound inside, whenever someone knocks on the door from the outside. This is the figure of the circuit;


The circuit is powered by a 9v battery, which should provide years of service. An on/off switch allows the device to be turned off if desired. It is suggested that the circuit be housed in a plastic box with a 9v battery holder. The door knock is detected with a piezoelectric wafer, glued to the inside bottom of the box, as shown below. The resistor R5 determines the knock sound sensitivity. The value shown should work in most cases. The box could be hard mounted to the door or suspended at about the middle of the door, by a string from the top of the door in such a way that the box rests against the door. Whenever a knock occurs, the dual comparator circuit will activate the beeper for a few seconds, and then turn off. You can use just about any type of beeping device you wish. Some people prefer the piezoelectric beepers with their higher frequency, while others may like the lower frequency electromechanical buzzers.

Electronic Doorbell Circuit With Counter

Posted by Micro System

on Friday, July 30, 2010

This is a design circuit uses a synthesized sound chip from Holtek, the HT-2811. This reproduces the sound of a "ding-dong" chiming doorbell. This is the figure of the circuit;


The Holtek HT-2811 is available from Maplin electronics in the UK, order code BH69A. The operating voltage must remain within 2.4 to 3.3 Vdc and standby current is minimal. The reset switch zeroes the count, and the 7 segment display is a common cathode type. To save power consumption the display can be enabled or disabled with a switch as shown in the above diagram. The count will still be held in memory. The envelope of the chime is set by the 220k, 330k, 3u3 and 4u7 resistors and capacitors. These values are the manufactures default values, but may be adjusted to alter the length and delay of the chime. The combination of the 2k2, 22k and 47u resistor capacitor network has a double function. It provides a de-bouncing circuit for the bell press and at the same time has a sufficiently long time constant. This ensures that anyone rapidly pressing the doorbell switch, only advances the count once. The 47u capacitor may be increased in size, if needed.

Doorbell Beeper Circuit

Posted by Micro System

on Tuesday, July 27, 2010

This is a circuit for front doorbell circuit. Whenever the front doorbell button is pressed, the hobby circuit will activate a loud beeper positioned in the basement. This circuit takes advantage of the fact that in many homes, the 24vac power transformer used to power the doorbell is located near the furnace in the basement. A box containing the electronic circuit could be mounted next to the transformer. This is the figure of the circuit;


A small bridge rectifier converts the 24vac into DC. The 100uF capacitor filters the pulsating DC from the bridge to form about 30v DC. Three diodes are wired in series with the door chime. Whenever the front door pushbutton is pressed, the door chime sounds. AC current drawn b the chime is routed through the three diodes shown. The two diodes in series forms a 2 volt voltage source, which is connected to an inexpensive opto-isolator. The 2 volts across the two diodes is sufficient to turn on the opto-isolator circuit, which routes DC to the gate of the ZVN4206 FET. When the doorbell chime sounds, the FET turns on a piezoelectric beeper, drawing power from the 30 DC supply. Make sure the beeper you use can handle the higher 30v voltage.

Smple Doorbell Alarm Circuit

Posted by Micro System

on Monday, July 26, 2010

This is a simple design circuit for alarm circuit. For operates this circuit, we nee a 9VDC battery. But the circuit itself will work from 5 to 15-volts - just choose a buzzer that's suitable for the voltage you're using. The standby current is virtually zero - so the battery life is good. This is the figure of the circuit;


If SW1 is fitted to a door every time the door opens the Buzzer will give a short beep. In an unattended shop - or reception area the sound of the beep will alert you to the fact that you have a customer. How long the output lasts depends on the values of R2 and C2. With the values shown - it will last for somewhere between 3 and 5 seconds. But by increasing these values you can achieve an output time of up to half an hour or more. So if you replace the Buzzer with a relay and use the relay to switch a Siren you have a Simple Intruder Alarm that you can fit almost anywhere. if you replace R2 with a 4M7 resistor you will increase the output time by a factor of about 5. If you replace C2 with a 470uF capacitor you will increase the output time by a factor of about 100. If you use both a 4M7 resistor and a 470uF capacitor together, you will increase the time by a factor of about 5 X 100 = 500. This should give you an output of around half-an-hour or more.