MECH 400
Microprocessor Supplemental Handout One
Circuits for Hardware Interface
Motor capacitors
A significant amount of electrical noise is generated at an electric motor’s armature, electrical
noise which will interfere with the behavior of the OOPic. To counteract this behavior, solder
three capacitors to each motor, as shown in figure 1. Solder one capacitor between the positive
and negative terminals, and one between each terminal and the motor can. It may be necessary
to use a file to remove the clear coat or paint from the motor can prior to soldering.
Figure 1: Motor capacitors
Digital input from switches and similar devices
To interface with mechanical switches, a pull up circuit as shown in figure 2 is required. The 5K
resistor is sized so that 1 mA of current will flow into the digital input port when the switch is
triggered. For reference, the I/O ports on the OOPic R board can source or sink a maximum of
20 mA. When the switch is open, the pin will read HIGH (+5 VDC), and when the switch is
closed, the pin will be pulled down to GND.
Figure 2: Digital input interface circuit
Solenoid control circuit
In order to control a solenoid (or any other on-off DC component) with the logic-level power
available on the OOPic digital output ports, a Mosfet is used. Connect the Mosfet and resistor as
shown in figure 3. When the digital output pin is high (+5 VDC) the Mosfet’s gate will be
saturated and electrons will flow through it from source (S) to drain (D), thereby driving the
solenoid.
It is also possible to use pulse width modulation at the gate to vary the current flow through the
Mosfet. This is the basis of the Mosfet speed controls and H-Bridges.
Figure 3: Solenoid valve control circuit