Servo Stimulator
For stimulation of position sensitive sensory cells in Aplysia, we wanted to have a flexible stimulator that would generate a slow, but repeatable stimulus. The classical solution is a small modified audio speaker or derivatives (Fig. 1, 2, 3, 4, 5 & 6), driven e.g. by a GRASS S88 Stimulator.
Figure 1: A rod is attached to a regular small audio speaker, supported by a tripod made from wire. The whole assembly is then mounted on a micro manipulator so it can be precisely positioned at the stimulated tissue.
Figure 2: The frame of the speaker is hard-soldered to brass rods, providing a ridgid support. The stimulation rod (aluminium) is isolated against the bath with a Sylguard coating at the tip and grounded via a fine wire. It glides in a short piece of brass tubing, held by the tripod. Both, tripod and rod, are glued to the speaker with Dow Corning 3140 RTV Coating.
Figure 3: A smaller version of this design uses Electronix Express part # 26SPPC as speaker. The speaker clips into a delrin holder which provides a 1/4"-20 threaded hole for mounting the stimulator onto a micro-manipulator. Despite the more compact size the stimulator is even more effective than the larger version.
Figure 4: This version uses a magnetic coil and shaft (originally used to lift the pen in a plotter) instead of the speaker. Thisresults in a larger movement at the cost of the fine control the speaker offers.
Figure 5: When the yellow coil is energized, the hinged, black front part is pulled against the resitance of the spring visible in the photo. A wodden stick fixed to the black front part pushes onto the stimulated tissue.
Figure 6: The stimulator is mounted on a small XY-stage for easy positioning.
However, none of the speaker based stimulators has a very long movement and stimulation parameters are difficult to control. We therefore looked for an alternative solution. It consists of a servo motor typically used in radio controlled toys. The servo drives a push rod - or rather push cable - that can be brought close to the stimulated tissue (Fig. 7).
Figure 7: Complete servo stimulator. On the right side is the servo motor connected via a push-pull cable with the stimulating tip on the left side. In the middle is the controller for the servo motor.
While the mechanical setup is very simple, the problem becomes to control the servo motor as it requires a PWM (pulse with modulation) signal. We build a small controller around a MicroChip PIC16F690 microcontroller. The microcontroller requires only few external components (Fig. 8 and 9) and can be quickly programmed to the specific experimental task. You can download the PCB layout here.
Figure 8: The circuit for the servo controller is build around a PIC 16F690 microcontroller. It requires only few additional components and is easily programmable. We added two switches and two potentiometers.
Figure 9: The printed circuit board completed with all components and mounted in a nice box.
The PIC microcontroller was programed with a PICkit 2 Development Programmer. The package is cheap and includes an assembler that was used to write the short program for the controller.You can download the source code here. The two potentiometers are used to set the two extreme positions. One button switches moves the servo motor from one position to the other. The second button does the same, only temporarily, to stimulate the tissue (Fig. 10).
Figure 10: The controler for the servo motor. We did not include an option to connect the controller to a PC, however, this could easily be realized with the PIC microcontroller.
A push-pull cable (in our case a piece of hookup wire sliding in PE tubing) is connected and servo and tip are mounted on magnets (Fig. 11).
Figure 11: The tip of the stimulator is mounted on a micromanipulator so it is easily positioned close to the stimulated tissue. The servo motor is mounted on a magnet and can be placed far a way from the preparation to reduce electric noise.