AV Amplifier Triggered HDMI Switch

AV Amplifier Triggered HDMI Switch

If, like me, you have an aging but trusted and favourite AV amplifier that works great but doesn’t switch HDMI, you may be interested in this simple project. It is based on another Instructable project [1], where the resourceful author hacked a manual HDMI switch for control using a microcontroller. I have taken the exact same approach, but have used the 12V trigger outputs of my Rotel RSP-1068 amplifier to operate the switch in place of the microcontroller.

The manually-operated switch described in [1] has 3 HDMI inputs and 1 output and has a single push button to toggle through the 3 inputs in turn; the selected input is indicated by the lighting of one of 3 LEDs.

Having hacked the switch and routed 5 wires through the case, exactly as [1], the problem reduces to one of repeatedly simulating a button push until the desired LED is lit.

My solution uses the venerable 555 timer IC in astable mode. In this mode, the IC generates a square-wave output continuously until its reset pin is brought to a LOW state. Figure 1 shows the circuit diagram. The input is one of the 12V triggers, which powers the IC and starts the oscillation, the reset pin (pin 4) is held in a HIGH state by the 10k resistor, and the output (pin 3) causes the transistor to ‘press’ the button on the HDMI switch (by grounding it) on every high part of the square wave. The timing between the ‘presses’ is set by the values of the large capacitor and resistors; the 0.47uF, 68k and 680k components giving about 0.5 seconds, which seems about right for the switch used.

The button is ‘pressed’ every 0.5 seconds until 2V appears on the LED of interest. This 2V switches on the second transistor which brings the reset pin to ground and stops the oscillation. Thus the HDMI switch stays switched to the input which corresponds to the lit LED.

Three of these circuits are required for 3 triggers and 3 HDMI sources, with each trigger associated with one of the 3 LEDs. The AV amplifier is configured to send a different trigger signal when each HDMI source is selected.

This all works fine but, although my AV amplifier does have 3 trigger outputs, I am already using one to switch on my power amplifiers, leaving only 2 triggers for selecting between 3 inputs. A solution comes in the form of a binary decoder chip. The 74HC238 IC takes 3 input signals, corresponding to the 3 digits of a 3-bit binary number, and sends an output signal on one of 7 output pins, corresponding to the ‘value’ of the input binary number. Figure 2 shows the truth table for the IC, and Figure 3 shows the IC pinout; both taken from the data sheet [2]. Figure 4 shows the circuit diagram for taking the two 12V trigger inputs and a permanent 12V (from my already-used trigger), and outputting one of 3 outputs; these outputs are connected directly to the trigger inputs of the 3 555 timer circuits (see Figure 1). The binary decoder operates at up to 6V, so the 12V triggers have to be reduced by the potential dividers on the IC input. Luckily, the resultant 5V outputs are sufficient to power the 555 timers.

So there we have it. Two selectable 12V triggers can operate the manual
HDMI switch to select any one of 3 HDMI inputs. I have built this project on a bit of veroboard and it works perfectly. What is more, if your AV amplifier can select any combination of trigger outputs, you can use 2 triggers to select between up to 4 HDMI inputs (00, 01, 10, 11) or 3 triggers to select between up to 8 HDMI inputs! Of course, you would also need a manual switch that will do this.

[1] https://www.instructables.com/Voice-Controlled-HDMI-Switch/ by ensuing, Apr 8th 2018

[2] https://www.ti.com/lit/pdf/schs147