Transcutaneous Electrical Nerve Stimulator (TENS)


A Transcutaneous Electrical Nerve Stimulation (TENS)
device is, put bluntly, a machine for giving electric shocks. The
author was prescribed such a device on loan by his orthopaedic
specialist. The unit has a large number of programmes, of which he used
only one. Measuring the signals at the output of the device in this mode
revealed damped oscillations at a frequency of approximately 2.5 kHz,
with a repetition rate of approximately 100 Hz.

Transcutaneous Electrical Nerve Stimulator Circuit

Transcutaneous Electrical Nerve Stimulator Circuit Diagram

How hard can it be to make such a device ourselves? The simple circuit uses a CMOS
555 timer to produce a brief pulse which feeds a 1:10 miniature
transformer. Together with a 4.7 nF capacitor the transformer makes a
parallel resonant circuit: the resonance leads to a considerable
increase in the output voltage. The pulse width can be adjusted using a
potentiometer, here shown combined with the on-off switch. Wider pulses
produce higher output voltages. Since a peak voltage of up to 200 V can
be produced, the transformer must have adequate insulation: Conrad
Electronics type 516260-62 is suitable. A low-cost phono socket at the
output gives reliable connection to the electrode cable.

The adhesive electrodes shown in the photograph (disposable and
permanent types are available) can be obtained from pharmacies and
medical suppliers. They generally have connectors compatible with 2 mm
banana plugs, and so it is possible to make up the necessary cable
yourself. To treat responsive parts of the body, such as the arm, the
potentiometer need not be turned up far to obtain the necessary
sensation. Less sensitive parts, such as the knee or foot, need a rather
higher voltage and hence a correspondingly higher potentiometer
setting.

Author: Klaus Rohwer – Copyright: Elektor Electronics Magazine

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