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IR-transmitter with an AVR processor ATtiny45

Properties of the remote control
Hardware and how it works
Building the remote control
Software

Properties

This remote control project offers the following opportunities:

Programmable for all desired devices, e.g. with one single button three or four different devices can be switched on. To do that the different codes
- have to be measured with the receiver described here,
- have to be analysed with the software described here,
- the resulting assembler source text for the desired remote control functions has to be copied to the source code file of the transmitter, and
- the assembled hex code programmed into the here described AVR hardware.
All devices that are in the receive area of the LED (with reflections e.g. in the same room) can be controlled, for which their code is known and programmed.
Selected and startet is a remote control sequence with the built-in keypad.
Begin and end of the transmit sequence can be monitored with a LED.
The supply comes from two batteries that can be switched on and off and so do not have any standby consumption.
If the devices to be controlled change, if an original remote control gets lost or is defect, this remote control can easily be reprogrammed (assumed you still have the monitoring data).

Hardware and how it works

The schematic of the remote control:
Scheme transmitter

In the center a processor of the type ATtiny45 was selected because it can be clocked with an Xtal (here: 2.4576 MHz). The clock fuse has to be set to an external Xtal with medium frequency. Derived from this Xtal is the modulation frequency of the IR LED LD271 at 38.4 MHz, but that frequency can be configured between 36 and 40 kHz by software.

The IR LED LD271 is not driven directly by a portpin but with a constant current source. This is built with two diodes, the power transistor BD439 and its emitter resistor and is set at 93 mA (= 0,7 V / 7,5 Ω). So, the operating voltage can be between 2,7 and 5 V without current changes and destroying the LED.

Voltage supply is with two batteries or accus, the power switch is not displayed here.

The processor programming signals SCK, MISO, MOSI and RESET are tied to a standard 10-pin ISP connector so that the whole remote control can be re-programmed. When re-programming via the Studio and a STK500 the battery supply should be switched off, the supply should come from the STK500 and VTG should be adjusted to 3 V. For correct function, the ISP connection should be removed after programming, because it interferes with the AD converter, and battery supply switched on.

Not displayed here is a red LED that displays start and end of the transmission sequence. This should be connected to MISO and via a 220 Ω to the positive operating voltage (output is active low).

Matrix voltages

On the input ADC1 of the processor the keypad is connected. The keypad produces an analogue voltage, depending from the key pressed. The matrix is dimensioned for commercially available resistors with 5% accuracy, for 8 bit resolution of the ADC and an as-linear-as-possible response curve. The calculation of the resistors was done with a calculation sheet in Open-Office-Format or as Excel sheet.

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Construction

Electronics, batteries, keypad, resistor matrix and the two LEDs fit well into a small plastic box ...

Built in

... the mounting of the single parts ...

... the processor part ...

... and the resistor matrix in detail.

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Software

The assembler source ode is available in HTML-Format here or as Assembler source file here.

The program works without timer, the timing of the signals is controlled with exact counter loops. All transmit loop are 64 clock cycles long, which a cycle duration of 26,04 µs. Only the AD conversion is interrupt controlled, and switched off during transmitting.

The tables are organised as follows:

The transmit sequence codes for the different keys (pStar, pNmbr, p0 to p9) are starting with the respective labels and end with one or two null bytes.
Active signals with activated ande modulated IR LED begin with bit 6 set and the number of repeats (example: 1+cH). If bit 7 is set additionally (example: 1+cHW) the number of active and inactive cycles follows with two bytes each, otherwise with a single byte each. The number of repeats is in the first byte (example: 4+cHW for four repeats). The four or two bytes following are the number of cycles, that is the number of IR LED switched on and off resp. the pause as long as if it would be switched on and off, with each cycle being 26.04µs long.
Each line must have an even number of five resp. three bytes long packages because otherwise the assembler would add a null byte and the sequence would end too early. Please take respective warnings of the assembler for serious.