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Infrared remote expander circuit Introduction

2017-08-09 00:47  
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This article describes an infrared remote control extender circuit description. The principle is simple there, we recommend that combine text carefully read the complete schematic.Light photons are received at IR1, this is an IR photo diode type SFH2030. A SFH2030F, which contains a daylight filter,may also be used instead of the  SFH2030 . The photo diode is reverse biased and when light strikes it, the energy of the IR signal releases additional charge carriers within the diode, allowing more current to flow. This current is amplified and converted to a voltage by the first CA3140 opamp, IC1. IC1 is wired as a current to voltage convertor, see below.


Parts List:
R1,R2: 5M6 RESISTOR (2)
R3,R5: 3k3 RESISTOR (2)
R4: 120k RESISTOR (1)
R6: 220R RESISTOR (1)
R7: 47k RESISTOR (1)
R8: 120R RESISTOR (1)
R9: 10k RESISTOR (1)
R10: 2K2 RESISTOR (1)
R11: 100R RESISTOR 1 W (1)
C1,C3,C4: 22n polyester CAP (3)
C2: 100u electrolytic 25V(1)
C5: 100u electrolytic 25V(1)
Q1 BC107 (1) alternatives, BC107A, 2N2222, 2N2222A
Q2 BC109C (1) alternatives, BC109, BC549
D1: 1N4148 DIODE (1)
D2: Red LED (1)
IC1,IC2 CA3140E opamp (1)
IR1: SFH2030: (1)
IR2,3: TIL38 (2) or similar.


At low frequency i.e. 50Hz the impedance of C1 is high, around 144k. The voltage gain of inverting op-amp IC2 is approximately R4 / R3, but at low frequency C1 is in series with R3 so the gain is now 120k / (3.3k 144k) or less than unity. Daylight or ambient light will change slowly over several hours, in frequency terms this signal would be millihertz or less and C1′s impedance will be megaohms.A signal from an IR handset will be modulated at around 36KHz. At this frequency the impedance of C1 is very low, around 200 ohms. This has little effect on the input impedance of the op-amp stage and voltage gain will now be R4 / R3 or about 34 times. The impedance of capacitor C4 also helps noise rejection as its impedance change will allow more signal to pass into Q1 base at high frequencies and much less signal at line frequencies.

In an ideal current to voltage convertor the output voltage would be the product Rf multiplied by the input current. The non-inverting input would be tied to ground. In the Mark 5 circuit the output voltage is iR1 or about 5.6 Volts/uA appearing at pin 6 of IC1. The current generated by the SFH2030 photo diode when receiving a signal from a handset several metres away is less than 50 nA and requires the extreme high input impedance to avoid shunting the signal. There are two reasons for using the CA3140, the first is its high input impedance, over 1000G. The second reason is that normally the non-inverting input would be at 0V when working from split and – supplies. In this single supply version the non-inverting input is returned to negative supply via R2. This can only be done with a Mosfet input, hence the choice for using the CA3140.IC1 converta all current from the photo diode IR1 into a voltage. Although the SFH2030 is most sensitive at infra red wavelengths, it will produce tiny currents from daylight and also the 50/60Hz noise fields from flourescent and mains lighting. To minimize this, C1 and R3 form a high pass filter, allowing a 30kHz and higher signals to pass but blocking low frequencies. The impedance of C1 increases with decreasing frequency being 31k at 50Hz. Daylight for example, produces a contstant luminence, changing slowly over several hours, to which the impedance of C1 is effectively infinite.The signal voltage from IC1 is now further amplified by IC2, gain being the ratio R4/R3 or 31dB. All opamps have a limit called the gain bandwidth product. The gain will fall to unity at the highest usuable frequency and be a maximum value at dc. Between these limits the gain falls with increasing frequency as shown in the bode plot for the CA3140 below:

ca3140 bode plot

Looking at the chart above, at 100kHz the maximum gain can only be about 30dB. However this is ample and boosts the received range of signals from a remote handset to the photo diode which have worked well up to 4 metres apart. Because R5 is returned to the negative supply a Mosfet input opamp must again be used. The output is again filtered by a high pass filter comprising C4 and the associated input impedance of Q1. R6, C2 and C3 provide decoupling for the IR preamplifier, C3 is in parallel with C2 because an electrolytic is not always a low impedance at high frequencies.The IR output stage is comprised of Q1 and Q2 and associated components. The output is arranged so that with no input signal, Q1 is on and Q2 off; the visible LED, D2 will also be off. With no signal the 47k resistor biases the driver transistor, Q1 into full conduction. Its collector voltage will be near zero volts and the output transistor Q2, which is direct coupled to Q1 collector will therefore be fully off. Power drain will be minimal.When an IR signal is receieved from a handset, the complete modulated signal will be amplified and fed via C4 into Q1 base. This is sufficiently strong enough to overcome the positive bias supplied by R7 and switch off Q1. This will happen many times a second, at the same frequency as the IR modulating signal sent by the handset. As Q1 switches off, its collector voltage rises to near full supply switching on Q1 and lighting the LED D2. Pulses of infra red at the same modulating frequency are then transmitted by the photo emitting diodes, IR2 and IR3. Because the signal is cleaner, (i.e. no daylight or 50/60Hz lamp fields included) then the series resistor R11 has been incresed in value to 100 ohms. The range from photo emitter diode to the equipment to be controlled has proved successfull at over 4 metres when powered from a 12 Volt supply. D1 helps to improve the turn off speed of Q1, thereby ensuring that the output waveform will be “squarer”. It can be omitted but the circuit will perform better if D1 is included. A simulated transfer characteristic is shown below:

AC Transfer Charcteristicac_transfer

The ouput is measured between Q2 emitter and ground. A simulated transient response is shown below. Three graphs are produced with excitations of 40,80 and 120kHz.

mk5 transientMk5 veroboard layout


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