Physiological Stimulus Isolator


The goal of this project was to build a small, cheap, light-weight stimulus-isolator unit (SIU) for student use in electrophysiology labs. Specifically:

The hardest goal to meet was not having batteries on the isolated output side. We tried photovoltaic optoisolators, but they could not supply enough power for the cells we were interested in stimulating. However, the photovoltaic isolators would work great for driving microelectrodes. More to follow on that topic at a later time.

We decided to try DC-to-DC converters which were fast, had low coupling capacitance, and produced over 30 volts at the output, isolated from a 6 volt supply. They are meant to be used to provide power to small circuits, but the isolation seemed good enough to use for an SIU.

The Circuit

The SIU is built around the Burr-Brown DCP010515D DC-to-DC converter purchased from Digikey. To get 100 volts we used three of them. If you only need 30 volts, use just one. The devices are rated to run at 5 volts input, but we ran them at 6 volts to produce more output voltage.

Stepping through the Circuit:

  1. The transistor at the input (along with the two diodes) limits, amplifies, and inverts the input pulse. The output of the transistor is a logic-level pulse suitable for driving the CMOS quad transmission gate.

  2. The CMOS transmission gate acts to ground the "synch in" pins of all three DC-to-DC converters. Grounding the synch pins turns them off. A high voltage at the input results in a low voltage at the output of the transistor, which turns off the transmission gate and turns on the converters. It is imperative that the wire from the transmission gate to the converters be as short as possible and minimize capacitance to ground. To much ground capacitance on this pin will slow down the internal oscillator and noise and poor response on the output. The output is thus on or off depending on a pulse at the input. Output amplitude is controlled by the potentiometer. Be sure to ground all inputs to the 4066 which are not used. CMOS gates tend to draw lots of current if their inputs are left unconnected.

  3. The three converters are connected in series to produce 90-100 volts. Note: The series output will hurt if you short your hand across it. Use caution with all high voltage sources. The 1 nF capacitors should be connected close to each converter. The converters are noisy if the capacitors are too far away. the 5 nF capacitor and 10 kohm potentiometer form a low-pass filter to absorb some of the 400 kHz noise produced by the converters.

Physical layout of the circuit

The circuit was placed in a plastic box to reduce capcitative coupling to ground. The transistor and 4066 should be placed away from the outputs of the converters for the same reason. A printed circuit board was produced to ease constuction and cut down on stray capacitance. The design file requires free, downloadable software to view, order, or modify. The board layout is shown below. Out+ and Out- go to the two ends of the output level potentiometer shown in the schematic.

  The circuit was built into a discarded WPI stimulus isolator chassis. all front panel controls were from the original WPI design.

More photos

The first three pictures are the original, hand wired model.

Analog stimulator

A circuit was designed to control the isolator. The circuit is very similar in overall function to the classic Grass stimulator. It produces one or two pulses with controllable frequency, pulse spacing/pulse delay, and pulse duration. It can be manually or remotely triggered. It produces a trigger output. A schematic and circuit board are shown. The board was designed with expresspcb software and can be viewed, modified and ordered using the free software. Note that the controls must be attached to the board as shown in the photographs below.