The latest addition to my collection of Infra Red (IR) Repeater circuits. The Mark 5 is a much improved version
of the Mark 1 circuit and has increased range and sensitivity. It is also immune to the effects ofambient light,
daylight and other forms of interference. In addition it works with IR modulation freuencies in the range 30 to
120kHz making the Mk5 circuit the best choice for compatibility with remote controls.
This time I have returned to "first principles" and built a wideband infra red (IR) preamp which
receives and re-transmits the entire baseband signal from a remote control handset.
It is designed to work with IR controls using 30-120KHz and should therefore work with just about any handset.
In addition I have separated ambient (surrounding) light from the modulated light used by a remote
handset. The major problem with the Mark 1 circuit is that it reacts to all light sources, ambient
light producing a continous signal from the IR photo diode and is amplified by the rest of the circuit.
I have published a modification to the original Mark 1 circuit, click here to view.
It is difficult working with Infra Red, you cannot see it, and it is difficult to measure. A major barrier
with this circuit was how to differentiate between daylight and an IR signal. Ambient light produces an
almost continuous signal, changing little over several hours. A signal from an IR handset
contains control pulses modulated with a carrier frequency (typically 36kHz) transmitted using an Infra
Red photo diode. My solution used here, is a simple RC filter formed by C1 and R3.
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.
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.
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:
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 Charcteristic
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.
Please note that the above waveforms are simulated using a perfect square wave input, with rise and
fall times of zero seconds. The output is measured between Q2 emitter and ground with a 200 ohm resistive
load. In the real world, the cable to the remote photo emitter LED's will contain both capacitance
and inductance. This will increase both rise and fall times of the output signal. As with the Mark 1
circuit I recommend using speaker wire or bell wire to be used to cable the remote photo emitters.
Note that the veroboard layout below only
includes the componets from the left of the schematic to C4, I had Q1 and Q2 on breadboard during
this testing phase.
Setup and Testing:
There is little to adjust in this circuit. First I suggest disconnecting the wiring to the emitters
IR2 and IR3. Switch on and D2 should be off. Aim a remote in the direction of IR1 and press any button
D2 should light and be seen to flash when a button is held on the handset and go off when unpressed.
If all is well reconnect the wiring to emitters IR2 and IR3. Without lenses, the light is quite
directional and so you will need to aim it carefully at the remote equipment you are controlling.
A digital camera, or camcorder can "see" into the Infra red range. This is useful to prove that
IR2 and IR3 are producing output.
Below is a picture of my veroboard layout for the Mk 5 IR extender using Ron J's excellent
veroboard images. Special thanks to Derek
Smith for checking the veroboard layout and pointing out one small error (which is corrected
Special Note:I have omitted Diode D1 in my prototype and also the
veroboard layout above, and the two images below.
Click the links below to view the actual veroboard layouts. The veroboard drawing above shows
the component site, the yellow circles represent the breaks on the bottom (track side). Component side (106k) Track side (97k)Note that this is reversed from component side.
For more help on vero layouts see this Practical Page.
If your circuit does not work, first check that your circuit is receiving power. Next compare
the voltages to my prototype below. These checks are all made with a digital multimeter with
a supply voltage of 12V DC. All checks are made with respect to ground (i.e. the back or negative
meter probe is always connected to the negative or 0V power rail).
A good tip from Derek Smith (UK, who had problems with poor noise immunity in this circuit.
Derek cured his fault by replacing the SFH2030 photo diode, the new SFH2030 provided much
better noise immunity. So, if your voltage levels are similar to my prototype above then
try replacing IR1.
If you still have problems with noise immunity check the supply voltage. Special thanks to Roch
who found out that his 12V power supply was actually running at 16V. After reducing the voltage
to 9V the problems disappeared for him. My original circuit ran happily from a 12V regulated supply.
If you build the mark 5 circuit please let me know the make and model of your remote
control. I will add it to the list of compatible handsets below:-
Kameleon One for all remote (URC-8060)
Maplins 6 way Audio/Video Switcher Hub order code L63AB
One for all remote
Panasonic DVD player model no N2OAHC000012
Pioneer DVD remote
RCA systemlink 8 A-V
Sanyo vhs remote
Sony RM1- V141A VTR/TV
Std Sky digi box handset