User`s guide

19 | P a g e
S Accelerometer Circuit
S-1 Technical Summary
The accelerometer circuit for the CABL can detect acceleration of half of a G- Force. The
circuit works by taking the signals produced by our Parallax mimsic dual axis accelerometer and
converting them to logical ones and zeros to be given to the control. The Accelerometer circuit
and the control circuit work together to accomplish this task. The two signals correspond to
acceleration along one of the axes. When the positive Y axis senses more acceleration the width
of the pulsed signal increases and vice versa. This is known as pulse width modulation.
What the circuit does with these signals is that it inverts them and then filters them across
a capacitor, in this case a one hundred micro-Farad. It was found experimentally that the process
of inverting, filtering, inverting, and then filtering again led to the voltage signal dropping to less
than two hundred mille-Volts when the pulse width shortened to a width corresponding to a forty
five degree angle of tilt in the accelerometer and consequently half of a G-Force. This drop was
taken advantage of by use of an NPN bipolar junction transistor. The N2222a transistor stops
conducting current once the voltage on the base terminal falls below five hundred mille-Volts.
When the Transistor collector is connected to a voltage through a resistor and another
capacitor connected to ground, the voltage is a logical zero when the transistor is conducting. As
soon as the transistor stops conducting the voltage through the resistor is passed on in the circuit
and is inverted twice to ensure a logical one.
To get the signal from both axes and in both directions the same process is used except
the signal is inverted before going through. So we have four channels and two with extra
inverters. This circuit functions very well.
S-2 Fault analysis
From studying our prototype, we see that it is not as sensitive as we would like. This may
be due to the fact that the accelerometer ended up in the bottle on its side as this did not come up
during the final preparation to install the circuitry. The Accelerometer could also be optimized
further by not requiring the final two inversions and it was not tried but the first inversions may
not be necessary.