Theatre Design & Technology - Summer 2001 - 41

Figure 1

Figure 2
CONTROL CIRCUIT
The key to the variability is the control circuit and when designing the control circuit, several things had to be taken into
account including:
* What were the power requirements of the solenoids?
* How long would the action have to take place?
* What was the power supply and its capacity?
* How was the timing for the solenoids to be adjusted?
* What range of adjustment would be needed for the timing?
* How would the performer initiate and stop the action?
Using the answers to these questions and considering
other factors, it was determined that the control circuit should
be a relatively low power design with standard components.
We chose a motorcycle battery to power the control system because of its compact size and because we had one from a previous project.
The base for the circuit design is a timer integrated circuit. Specifically, we used the LM555 timer. This is a common
IC that is manufactured by several companies such as National
Semiconductor and Texas Instruments. It has been in use for
several years and can be found at Radio Shack or other electronic component retail outlets. There are many example circuits and application notes available from the different
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manufacturers and from different electronics sources. The IC
is simple to configure, depending on the operation desired.
The IC's basic function is to output a high/low signal at a predetermined rate of speed. Two resistors and a capacitor that
are connected to the IC configure the rate of speed.
In order to maintain the design goal of future usability for
the control circuit, it was decided that the timer circuit output
would use 12-volt relays with 10-amp contacts. Therefore the
timer IC would drive a transistor that would drive a relay coil
and the relay contacts would be used to control the solenoids.
Another reason to have the timer IC drive a transistor is that
the IC has a limited power output rating. Furthermore, using
the output of the timer IC to drive the transistor allows the
timer circuit to operate at a lower voltage than the relay coils.
By giving the circuit a lower operating voltage and using relays
with a wide operating range, we helped to insure proper functioning as the battery was drained.
Another issue that had to be considered was the design
goal for the project of operating four different cylinders, each
operating with different timing characteristics. This would require four different timer circuits. In order to conserve space
and wiring the LM556 timer IC would be used. The LM556
timer IC is a single IC that contains two complete LM555 timers. With all of these choices made, a general timer circuit was
developed. Figure 3 shows the general layout for one of the
single timer circuits. Then simply repeat this four times and
adjust the connections to the LM556 timer IC for the appropriate pins to create the entire circuit,
The output of the timer IC, simply stated, switches from a
high voltage level to a low voltage level. The end result at the
transistor is that at the high level, the transistor turns on and
connects the relay to ground. At the low level, the transistor is
off and acts as an open switch. As stated previously, the time of
this switching is controlled by the selection of two resistor values and a capacitor. In the circuit diagram these components
are labeled R1, R2, and C1. The formulas for calculating the
times are:
t1 = 0.693 x (R1 + R2) x C1
t2 = 0.693 x R2 x C1
This formula is given in the application notes and data sheets
for the IC. For our purposes, we needed to be able to adjust
the times, so R1 and R2 were potentiometers in series with resistors instead of just fixed value resistors. Using a spreadsheet
we were able to determine appropriate values for the pots that
would give us a wide range of timing choices. The capacitor
was left as a fixed value capacitor. The potentiometers were fitted with locking, calibrated knobs so that once the timing was
configured, the potentiometers could be locked and a scaled
value could be recorded.
In order to increase the functionality and flexibility of the
control circuit, the contacts and the remaining coil input for each
relay was brought out to terminal strips mounted on the control
circuit enclosure. Also, an additional relay was added to the cir-

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