Hardware Reference
In-Depth Information
waveform goes up and down rapidly it will start to charge up C3; it only gets more charge
when the output of the first amplifier exceeds the voltage on C3 so this capacitor remembers
the peak voltage of the audio signal. That is all well and good but you need some way of for-
getting a peak signal that happened some time ago and so R3 discharges the capacitor at a
slower rate. The result is that the voltage on C3 represents the peaks of the signal or, as we
say, it is an envelope follower. The value of R3 determines how quickly the envelope decays.
This envelope voltage is fed into the second op amp. Here you have no feedback, and you just
use the open loop gain. The negative input is fed by a voltage set by a knob or pot VR2; this is
a threshold voltage. If the envelope voltage is above this then the output goes crashing up to
the supply rail of 5V. If, however, the envelope voltage is below this threshold then the out-
put gets put firmly at zero volts or ground. This digital signal is too big to be fed into the
PiFace board so it needs cutting down with R4 and D2 to make it a 3.3V signal suitable to
dive the sequencer. D2 is a special sort of diode known as a
zener diode;
it starts to conduct at
a set voltage. You can get these diodes that conduct at all sorts of voltages; you want one here
to conduct at 3.3V or, as it is often written,
3V3.
The ground is shown by the hatched symbol
at the end of R1. All the points with this symbol must be connected together.
Building the Circuit
So what you need to do as the final step is construct this circuit. I much prefer making circuits
on strip board and not solderless breadboard. The problem with breadboard is that it can make
poor or intermittent contact which means that you could appear to have wired it up correctly
but it is not. Therefore you can waste a lot of time just jiggling the components around hoping
this will make it work. A small piece of veroboard or prototyping strip board is all you need. You
can also use sockets for the integrated circuits, which means you can reuse them or replace
them if they are damaged. The physical layout of the circuit is shown in Figure 3-8.
Figure 3-8:
The physical
layout of the
beat driver
circuit.
