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The kit consists of two parts : a
transmitter & a receiver.
The transmitter sends out radio wave of frequency around 300 MHz modulated by a low frequency tone generator.
When this RF signal is received by the super-regenerative AM receiver, a PLL tone decoder IC checks for valid tone.
If the tone
matches with the decoder IC (NE567) VCO
frequency, a miniature relay is
actuated.
Operating mode : momentary
Click here for
NE567 datasheet (pdf)
BASIC OPERATION
When the
button on
the transmitter is pressed, the miniature relay on the receiver will be
actuated and the LED is on. The
relay de-activates (and LED off) when
the button is released.
In order for the receiver to respond correctly to the command of the
transmitter, the following conditions must be met :
(1)
The receiver must be tuned to the same frequency as the transmitter ‘s
carrier frequency
(2) The
frequency of the tone generated in the transmitter must fall within a frequency
range set by the preset (VR)
of IC2 (NE567).
(1)
The
TRANSMITTER
The
logic gates IC1A & IC1B are not used in this circuit and their
inputs are permanently grounded. The other two gates IC1C & IC1D
form a square wave oscillator which generates an audible tone of about 1 ~ 2
KHz. Transistor Q4 is
a RF oscillator that generates high frequency radio wave of around 300 MHz (UHF
band). The output of the tone
generator (pin 10) is used to “modulate” the RF carrier frequency
which is transmitted to the remote receiver when the button is depressed.
(2)
The RECEIVER
The
first stage of the receiver unit is a super-regenerative AM
receiver comprising Q1, L2 and several other passive components.
When it receives the signal from the transmitter, it extracts the
tone component of the signal which appears at the junction of resistor R7
and R8. This signal is much too weak to be of any use. The amplifier (Q2)
is therefore included to amplify the signal to a higher level of
around 100 ~ 200 mV. This signal which appears at the collector of transistor Q2
is then ac coupled (by C7) to the input of the tone decoder IC (IC2).
Next we shall discuss the operation of the phase-lock loop (PLL)
tone decoder NE567 (IC2)
(3) TONE
DECODER & RELAY DRIVER
The
NE567 is a phase-lock loop (PLL) IC which is designed
mainly for tone decoding application. The IC has a VCO (voltage-controlled
oscillator) which runs at a frequency fo determined by the components VR, R3 and C5.
If the frequency of the input signal (at pin 3) falls outside the range
of +/- 14% of fo,
the output (pin 8) is open (or floating) . However, when the input signal
is within the range, the output (pin 8) is grounded thereby actuating the relay
& lighting up the LED.
