35KHz Magnetic Radiation Remote Control

Posted by Author makecircuits

35KHz Magnetic-Radiation Remote-Control

There Short-range 35KHz operation, single-channel unit

Simple circuitry, no outer antennas required

Transmitter circuit diagram:

Transmitter parts:

R1_____________68K 1/4W Resistor

C1______________4n7 630V Ceramic or Polyester Capacitor

C2__________60-80pF 63V Ceramic Trimmer

C3____________100΅F 25V Electrolytic Capacitor

Q1____________BC337 45V 800mA NPN Transistor

Q2____________BD139 80V 1.5A NPN Transistor

L1_________________ 500 turns on a 10mm. diameter, 10cm. long ferrite rod.

Enameled wire diameter: 0.2mm.

The tap is made after 200 turns, ground side

P1_____________SPST Pushbutton

B1_____________6-9V Battery (4 to 6 AA 1.5V Cells in series, see Notes)

Receiver circuit diagram:

Receiver parts:

R1,R3___________1M 1/4W Resistors

R2,R4__________47K 1/4W Resistors

R5____________330K 1/4W Resistor

R6,R7__________68K 1/4W Resistors

R8____________180R 1/4W Resistor

R9____________100R 1/4W Resistor

C1____________470pF 63V Ceramic Capacitor (See Notes)

C2_____________10nF 63V Polyester or Ceramic Capacitor

C3____________100΅F 25V Electrolytic Capacitor

C4,C5_________100nF 63V Polyester or Ceramic Capacitors

C6______________1΅F 63V Polyester, Ceramic or Electrolytic Capacitor

D1_____________5 or 3mm. Red LED

Q1,Q2,Q3______BC549C 25V 100mA NPN High-gain Low-noise Transistors

Q4____________BD328 30V 800mA PNP Transistor

L1_________________ 700 turns on a 10mm. diameter, 10cm. long ferrite rod.

Enameled wire diameter: 0.2mm.

The tap is made after 350 turns, i.e. at the center of the winding

BZ1___________Piezo sounder (incorporating 3KHz oscillator, optional, see Notes)

RL1______________5V DIL Reed-Relay SPDT or DPDT (Optional, see Notes)

B1_______________3V Battery (2 x 1.5V AA, AAA or AAAA Cells in series or 1 x 3V Lithium Cell)

Device purpose:

This unit can be useful as a short-range, single-channel remote-control. When the pushbutton in the transmitter circuit is briefly activated, the LED D1 in the receiver illuminates and an optional beeper or relay can be operated.

Circuit operation is based on a non-modulated 35KHz frequency carrier transmitter, and on a high-gain tw1o-stage 35KHz amplifier receiver, followed by a frequency-voltage converter and DC load driver.

Outstanding features for this design are as follows:

  • No outer antenna is required on both transmitter and receiver sections, due to the very low frequency operation. The antennas are 10mm. diameter, 10cm. long ferrite rods supporting the coils.

  • Unlike Infra-red remote-controls, this unit operates through the walls etc.

  • No radio-frequency interference in spite of simple circuitry.

  • The receiver operates at ultra-low voltage supply (3V) and standing current (100΅A): in this manner it can be left in stand-by mode for years before a battery replacement is needed.

Snags are: the short-range operation (about a medium-sized apartment), the high number of windings for the coils and the high current drawn by the transmitter.

Luckily, this latter snag is compensated by the fact that only a short pulse from the transmitter is needed to operate the receiver. Therefore, if the transmitter is not operated continuously, its battery should last long.

Transmitter circuit operation:

Q1 and Q2 are wired as a Darlington pair to obtain the highest possible output from a Hartley type oscillator. C2 must be trimmed to obtain the highest sinewave output (best viewed on oscilloscope). In the prototype the sinewave amplitude measured at C1 leads reached 800V peak-to-peak at 9V supply and 450mA current.

Receiver circuit operation:

Q1 and Q2 form a tw1o-stage linear amplifier. Therefore, the small 35KHz signal picked-up by L1 is highly amplified by these devices and feds Q3 wired as a pulse-to-DC converter.

When the input signal reaches Q3, the collector voltage of this transistor goes low, thus activating the LED D1 (or the optional beeper or relay) by means of Q4.

Stand-by current is only 100΅A. Current drawing is about 10mA when the LED is on and about 20mA when a relay is activated.


  • Q2 in the transmitter should have a small heatsink.

  • A good compromise is to use a 6V supply for the transmitter (four 1.5V AA cells in series). In this case current drawing is 300mA.

  • Needing a shorter range operation, Q2 in the transmitter can be omitted. Therefore, the emitter of Q1 will be connected to the tap of L1 coil. In this case the circuit could be powered by a 9V PP3 alkaline battery, drawing about 100mA current.

  • The receiver must be tuned to the transmitter frequency. Starting with a 470pF value for C1, you should try to modify its value by means of small capacitors wired in parallel to it, in order to obtain the highest AC voltage output at Q2 or Q1 collector (best measured with an oscilloscope). C1 value might vary from about 400 to 800pF.

  • Do this setup with transmitter placed 4-5 meters away from receiver. During setup it is wise to temporarily connect the transmitter to a 6 or 9V regulated power supply, in order to save batteries.

  • A small DIL 5V reed-relay was used in spite of the 3V supply of the receiver. Several devices of this type were tested and it was found that they switch-on with a coil voltage value comprised in the 1.9 - 2.1V range. The coil resistance values varied from 140 to 250 Ohm.