WEEK 10

Basic Circuitry of EMS Devices

In the muscle stimulator circuit an astable multivibrator, the component named IC1 7555 in the circuit diagram, generates pulses at around 100 Hz. Astable multivibrators are unstable, in other words, they never stay in one state. They are used in stimulators because they are oscillators that can create pulses trains at a desired rate. A potentiometer, labeled VR1 in the diagram, acts as a control knob, allowing the user to alter the intensity of current going through the electrodes. LEDs can be placed on the circuit to indicate the intensity level of the output current. Increasing the resistance of the 1.8K resistor labeled R2 can increase the current intensity. The current then goes through the reverse-connected transformer labeled X1, which down-converts the current to safe levels. The transformer must be reverse connected, meaning the secondary coil goes to the collector and ground, while the primary coil goes to the electrodes. This is done to prevent high level current from going through the electrodes and shocking the patient. Two transistors (T1 and T2) act as the buffers and invert the polarity of the pulses. They also drive the transformer and it protect it against high voltage leaks in case of false usage.

The timer uses a monostable multivibrator, the IC NE555. Monostable multivibrators have a single stable state and an unstable state. The unstable state can be triggered and the multivibrator will return to its stable (off) state after a certain amount of time. This characteristic makes it an ideal component for a timing device. The capacitors in the diagram are used to prevent unsafe currents.

EXTRA NOTE: The two parts of the device do not work synchronously. The timer and stimulator circuits usually have separate switches and voltage sources. The user is usually allowed to pick how long they would like to the stimulation to last. Thus, it is best to design a system where one switch triggers the stimulator and another triggers the timer. That way, the user can activate the timer switch, select the time of stimulation, and then activate the stimulator to begin treatment.