Date of completion: 2012
These are two RF devices to solve a problem! I live about 100 metres from the church where my wife is the vicar. The burglar alarm sounds from time to time, and, particularly at night, it is rather a nuisance to have to go and investigate it (it is almost always a false alarm!) The trouble is that it is difficult to distinguish the sound of the church alarm from car alarms, which also sound at fairly frequent intervals. This transmitter/receiver pair is designed to indicate if the sound is indeed coming from the church tower.
Design
The RF frequency used is 433MHz (within a public band in the UK) using a transmitter/receiver pair cheaply available on Ebay and HT12E/HT12D encoder/decoder chips, also readily available. Both are powered from 9V mains adapters feeding through 7805 voltage regulators. They each have an aerial attached as vertically as possible of length 17.4 cm (quarter wavelength), which are JUST sufficient to work over the distance involved here,
The transmitter (mounted in the church ower) has a small microphone to detect the sound of the (very loud) alarm (the church bells are also picked up, but these are easy to distinguish aurally.) Its signal is fed through an op amp to amplify the microphone signal, and then another op amp without feedback to cause saturation (i.e. with its very large gain.) The square-wave output is fed via a switching transistor with a low-pass filter into a 555 monostable circuit, set to about 10 seconds. Another 555 timer is used as a multivibrator with a time period of about 2 seconds but an "on" period of only a few milliseconds. This second 555 drives the encoder to enable transmission for a very short time every 2 seconds. It transmits a signal indicating whether the monostable is on or not.
The receiver is simpler, feeding the "transmission received" signal from the decoder to a 555 monostable set to about 1 second, in order to flash a red LED indicating transmission received (every 2 seconds.) As the decoder latches its output, a green LED is simply connected to the data output indicating the content of the last transmission.
The receiver has now (2017/2018) been integrated into a GPS clock, so I have no photos for you!. See "GPS Clock" to learn about the latest development.
The transmitter (mounted in the church ower) has a small microphone to detect the sound of the (very loud) alarm (the church bells are also picked up, but these are easy to distinguish aurally.) Its signal is fed through an op amp to amplify the microphone signal, and then another op amp without feedback to cause saturation (i.e. with its very large gain.) The square-wave output is fed via a switching transistor with a low-pass filter into a 555 monostable circuit, set to about 10 seconds. Another 555 timer is used as a multivibrator with a time period of about 2 seconds but an "on" period of only a few milliseconds. This second 555 drives the encoder to enable transmission for a very short time every 2 seconds. It transmits a signal indicating whether the monostable is on or not.
The receiver is simpler, feeding the "transmission received" signal from the decoder to a 555 monostable set to about 1 second, in order to flash a red LED indicating transmission received (every 2 seconds.) As the decoder latches its output, a green LED is simply connected to the data output indicating the content of the last transmission.
The receiver has now (2017/2018) been integrated into a GPS clock, so I have no photos for you!. See "GPS Clock" to learn about the latest development.
Circuit Diagrams
Tiny CAD (.dsn) files: Alarm Transmitter Alarm Receiver
No comments:
Post a Comment