Hardware Reference
In-Depth Information
Figure 11-7:
he schematic of
the beat driver.
his uses two op amps which conveniently come in one package. he signal passes through
C1 to remove any DC component and then into a pot so that you can set the level into the
ampliier. his irst op amp is conigured just like the previous example as a non-inverting
ampliier, only this time there is a capacitor across the feedback resistor. his means that for
low frequencies the gain is determined by the value of resistor R2. But as the frequency
increases, the capacitive reactance of the capacitor shorts out the feedback resistor to lower
the gain. So in this section you have combined a low-pass ilter with an ampliier.
he output of this ampliier is passed through a diode. his is a component that will only let
electricity low in one direction, in this case from the ampliier into capacitor C3. So as the
waveform goes up and down rapidly it will start to charge up C3; it only gets more charge
when the output of the irst ampliier exceeds the voltage on C3 so this capacitor remembers
the peak voltage of the audio signal. hat 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. he result is that the voltage on C3 represents the peaks of the signal or, as we
say, it is an envelope follower. he value of R3 determines how quickly the envelope decays.
his envelope voltage is fed into the second op amp. Here you have no feedback, and you just
use the open loop gain. he 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 irmly at zero volts or ground. his 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.
he ground is shown by the hatched symbol
at the end of R1. All the points with this symbol must be connected together.
