ES-31      UHF Radio Wave Remote Control Experiment
                      (momentary mode )   ( Transmitter : DC 9V,    Receiver : DC 12V)

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 will be de-energized (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.

For more info on NE567  and he principle of PLL, please visit the pages "Datasheets" and "Projects & Links"