Lukas Bauer

Adding a discrete "power on" code to Topfield's TF5000PVR (or any other device)


German Version


This article shows how to add a second IR decoder that will power up any device explicitely only when it was in standby mode upon reception of a "power on" code that can be useful in macro sequences of programmable infrared remote control units.


Use of discrete "power on" codes


Many multi device IR remote controls support macro sequences that will, for example, activate a DVD player by switching the TV to the correct input and powering up the device. To prevent the device from being powered down when it was already on, the remote conrol should send a discrete "power on" code rather than a "power toggle" code that will change the on/off state. While most original remote control units are only capable of sending the "power toggle" code, many modern devices also understand discrete "power on" and "power off" codes. Unfortunately there are devices, like Topfield's TF5000PVR(t), for which a "power on" code does not exist.




Make sure your device is not connected to power when opening it. Any modification of electric devices poses the risk of electric shock, damaging your device, heat, fire, injury, death, losing your warranty, and other dangers. The author rejects responsibility of any kind - it's entirely your own risk.


Adding hardware to decode a "power on" code


Reception of infrared commands is ususally done using two components. An IR receiver converts the optical signal into an electrical signal, amplifies it, filters the relevant frequency, and outputs a digital signal that is a sequence of coded pulses. The second stage is a decoder that can recognze the different IR commands from the coding and modulation of the pulses. There is a variety of coding schemes used by different manufacturers. Historically decoding was done using integrated circuits like the SAA3049 for Philips' RC-5 code. Today, decoding is usually done by microcontrollers. In the TF5000PVR, the IR receiver is a TSOP4838, and decoding is done by the frontpanel processor.


By adding a second IR decoder listening in parallel to the output of the IR receiver it is possible to decode additional IR commands. When remote control and decoder are programmed correctly, an output of the decoder will become active when the selected "power on" code is received. This can be used to power up the device. An elegant way is to emulate a key press of the "standby" button (on the device, not the remote) by shorting the contacts of the key while the IR decoder signals the reception of the "power on" code IF the device is in standby mode.


In the TF5000PVR, an internal signal "p_on" indicates whether the device is on or off (1=off). By using a series connection of two analog switches, the power indicator and the pulse from the IR decoder can be used to emulate a key press only when the device was in standby mode:

Image 1: Simplified diagram of the extension for discrete "power on" codes



To add a "power off" code as well, a second signal from the IR decode could be used (not required for the TF5000PVR, a "power off" code exists):

Image 2: Simplified diagram of the extension for "power on" and "power off" codes



Selecting an IR decoder


As the IR receiver contains filters for the specific frequency used in the existing IR codes of the device, the new "power on" code and its decoder should be selected to match this frequency. A deviation of ±2kHz may be acceptable but results in a reduced sensitivity, i.e. range. In the TF5000PVR, a TSOP4838 receiver designed for 38kHz codes is used. The most universal solution is a freely programmable IR decoder that can "learn" any code. In this case, for optimum range, the learned codes should match the receiver's frequency.





2010 cost



protocols decoded



USD 9.95



"learns" any 2 codes


Van Ooijen

EUR 5.25



38kHz Sharp, 36kHz RC-5



USD 8.00



40kHz SONY


The SIS-2 has been selected for the TF5000PVR example. After momentarily shorting its "/learn" pin to ground, it will learn the two IR codes sent to it (each code must be sent four times). If the "mode select" input is left open, each code will generate an active high pulse at one of its two outputs while the code is being received.


Example implementation for Topfield's TF5000PVR


The following schematic and photos show an example implementation for the TF5000PVR. The additional circuit has been built in an SMT like style (using SMD LED's, but wired IC sockets with their leads spread flat). The small PCB fits onto the front panel. The additional 3 resistors on the front panel serve the different purpose of reducing the intensity of the Topfield's three status LED's.


Image 3: Schematic of the circuit for Topfield's TF5000PVR(t)




Image 4: Front panel of TF5000PVR(t) with the additional PCB



Image 5: Intermediate and final PCB




Teaching codes


For teaching codes to the SIS-2 decoder, it is recommended to operate the decoder outside the device to avoid dangers of electric shock. By temporarily attaching a TSOP4838 IR reciver, or by using powering the whole Topfield frontpanel from a safe 5V power supply, it is possible to reduce risks.


Considerations for adapting the circuit to different devices


In principle, similar circuits can be used to add a discrete "power on" IR code to any device. Due to the varieties af devices it is impossible to present a general solution here.


The most obvious problem is to localize a "power indicator" signal in your device that tells the circuit whether the device is already powered up or in standby mode. In most cases there are switched 3.3V or 5V power supplies on the main PCB that are off (0V) in standby mode. Depending on voltage levels and polarity of the power indicator signal, some modifications of the circuit may be necessary, e.g. using an analog switch that is "on" when the indicator signal is "low" if this indicates standby (SW3 in the DG403).


© 2010 Lukas Bauer


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