Tube amplifiers are audio devices that are at the higher end of the price range. Despite their high price, most devices only allow operation directly on the device. There are remote controls for very few.
This is where the following project comes in. Three basic functions are realized in this self-built remote control:
- Power on/off
- Volume control
- Mute
There is no need to intervene in the amplifier. You will find out later in the text how the receiver is installed in the home audio system.
Effort
Suitable for: Beginners with some soldering experience and BASCOM knowledge
Cost of receiver: approx. 40 € without housing
Cost of transmitter: approx. 7€ without housing
Expenditure of time:
- Time required to assemble and wire circuit boards: 3 hours
- Time required to produce circuit boards: 2 hours
- Software installation: 1/4 hour
This is required:
- reichelt shopping baskets without mechanical parts (housing, screws, etc.):
Parts list for the receiver and parts list for the transmitter - Tools: Soldering iron, screwdriver, side cutters, devices for producing circuit boards (exposure device, etching system) or assembly on perforated circuit boards
- AVR USB programming adapter
- BASCOM compiler (freeware version)
- Sprint layout viewer (freeware version)
Safety notice
The assembly and construction of the circuit are harmless. The commissioning does not pose any particular danger either, since the power supply unit used has complete electrical isolation. Only the wiring of the switching connection should be checked by a qualified electrician, as this is a switching device with 230V mains potential.
Description RC5 remote control code
The most common coding method for IR control in Europe is the RC5 code. This was originally developed by Philips and is able to send 2048 different commands: 32 group addresses with 64 commands each. Each device has its own address. A complete message has a length of 14 bits and consists of the following bits:
- Two start bits to control the AGC (Auto Gain Control) levels in the receiver IC
- A toggle bit indicates that a new key is pressed
- Five system address bits
- Six command bits
The toggle bit changes its value each time a new key is pressed to distinguish between pressing and holding down a key. The five address bits follow the toggle bit and indicate which device it is. Finally, the six command bits contain the control information. The RC5 code uses two-phase coding with a carrier frequency of 38 kHz. A low/high combination represents a logical “1” while a high/low combination represents a logical “0”. The length of each bit is 1.778 ms, and a complete message is 24.889 ms long.
Infrared remote control selection
The remote control receiver is operated using an RC5-compatible remote control. You either build it yourself – the instructions for this are part of this article – or you use one of the many universal remote controls that can be bought cheaply everywhere. They all support the RC5 protocol. Philips TV remote controls work best. Simply select a device from the code list – the control codes used by the receiver are supported by all remote controls:
| On/Off | Command = 12 | POWER button |
| Volume up | Command = 16 | VOL + button |
| Volume down | Command = 17 | VOL – button |
| Mute | Command = 13 | MUTE button |
If the existing remote control sends other codes, you can connect a PC with a terminal program, such as HTERM, to the receiver. The optional RS232 interface (parameter: 9600, 8, n, 1) on the receiver is used for this. The terminal program is able to display the code sent by the remote control. With this information, you can customize the BASCOM source code with just a little programming knowledge.
Placement of the receiver on the audio system
It doesn’t take much to be able to remotely control the tube amplifier. All you have to do is loop the receiver into the amplifier’s external wiring.
Audio signal
The remote control receiver has a pair of cinch sockets for input and output. For remote control of the audio signal, insert the receiver as an intermediate device into the cinch connection between your audio source (CD, record player, etc.) and the tube amplifier. Make sure to plug the audio source into the input jacks and the amplifier into the output jacks, otherwise the potentiometer will rotate in the opposite direction. If this is the case, the audio signal will not be played back as intended.
230V switching output
The 230v switching output switches the power supply on or off for all poles. In order to prevent the receiver housing from becoming too large, no sockets were installed. The power connection is made via plug-in couplings. Since a tube amplifier is usually built according to protection class I, a two-pole CEE16/7 connection – also known as a Euro plug – is completely sufficient.
If, contrary to expectations, the amplifier has a protective earth conductor, the corresponding plug-in coupling must also be three-pole. During construction, make sure that the protective conductor is connected accordingly. If you use a metal housing for the receiver, all metal parts must also be connected to the protective conductor.
Structure and description of the hardware
It is advisable to use the circuit boards included in the download package. The use of breadboards is only possible to a limited extent, as some of the larger components do not fit into the 2.54 mm grid.
You start assembling the board with the lowest components: jumpers and resistors.
Recipient
There are four assemblies on the receiver circuit board:
- +5V and +12V power supply
- H-bridge for volume control motorized potentiometer
- Relay switching stage for the 230V power supply of the tube amplifier
- RS232 level converter (optional)
The control is powered by a 12V power pack with direct or alternating current. The current consumption is a maximum of 200mA.
The motor is controlled via an H-bridge. Counterclockwise rotation corresponds to the “Quieter” function, clockwise rotation to the “Louder” function. If the silent button is pressed, the motor runs to the left stop. The audio signal is routed via the potentiometer using cinch sockets. In order to avoid noise from the motor, high quality of the three interference suppression capacitors is essential.
The switching stage for the power supply of the tube amplifier can be loaded with a maximum of 3A. When choosing the fuse, take into account the sometimes very high inrush current of tube amplifiers. In most cases a 3A slow blow fuse should be sufficient. With some devices, however, the use of a 4A fuse is essential.
If you have misplaced the remote control, you can also adjust the volume manually on the receiver. You can also operate the switching output manually using a built-in button on the front panel.
The three LEDs on the front panel provide the following information:
| LED yellow | 12 V operating voltage applied |
| LED green | Switching output switched on |
| LED red | Infrared signal is being received |
The RS232 converter on the circuit board allows you to connect the receiver to a PC with a terminal program, as already described. This shows the codes sent by the remote control. The converter is not essential for the functionality of the remote control, but can also be omitted if necessary.
Transmitter
If you would like to build the transmitter yourself, it can be installed on a perforated board without any problems. In this case, the board layout provided is of course only a suggestion. The transmitter is operated with a 9V block battery. In standby mode, the electronics draw about 20 nA. In stand-by mode, the battery is not discharged under normal circumstances. If a key is pressed, the processor starts and first switches on the power supply. The transmitter then operates until the processor has transmitted the selected RC5 code. After that it is switched back to the standby mode with a delay of 500 mS.
In transmission mode, the battery is loaded with about 20 mA. However, since this only happens for a very short time, the battery is guaranteed to have a long service life.
The specified type does not necessarily have to be used as the infrared transmitter diode. Any IR diode that transmits in the 950-980 nM range will do.
Description of the software
The software, both for the transmitter and for the receiver, is written in BASCOM AVR-Basic. To compile the source code, you only need the freeware version of the compiler. You also need an AVR ISP programming adapter to transfer the compiled source code to the two microcontrollers. Both programs are linked above. The download package already contains the compiled files with the extensions .bin and .hex, which you can load directly into the microcontroller.
Receiver
The source code of the receiver software is self-explanatory: the variable names are chosen so that the function can be recognized directly. Lines 31 to 35 in the source text are particularly interesting. In lines 31 to 34 the remote control codes are assigned to the functions. If the remote control used does not adhere to the RC5 codes of the Philips assignment, you can make adjustments here. Finally, in line 35, the runtime of the motorized potentiometer can be set if a volume button has been pressed.
Transmitter
Lines 29 to 32 also specify which code is assigned to which key. Here, too, you can adjust the assignment individually, depending on your preference.
Conclusion
The convenience of a remote control contributes significantly to relaxation when listening to music. You don’t have to keep running to the amplifier to adjust the volume. The advantages outweigh the disadvantages of the lack of exercise. Nothing stands in the way of an appealing musical experience.
Images: reichelt elektronik
