Part 1. Construction of the Stress Meter

Version 1.0, April 1998

Background

The idea for a biofeedback device (we have called it a 'stress meter') is to have some form of electronic monitor to measure the relaxation or stress of the subject as he or she tries to relax whilst watching some form of meter to determine how relaxed they are, and at the same time trying to find out what produces greater relaxation (lower stress), and so change the meter reading. An individuals heart beat rate becomes slower with lowered stress levels, that is greater relaxation, and skin temperature increases slightly, since circulation is better when we relax, and decreases when we are stressed. Our particular meter measures the skin resistance since when we are stressed we also tend to sweat more, and this lowers the skin resistance from the perspiration. A resistance of a megohm or more is given by very dry skin, as we sweat the resistance drops to a few hundred kilohms.

The inspiration for this project came from a very useful little booklet sold in Maplin Electronics, "Easy PC Interfacing" by R.A.Penfold, Bernard Babani Ltd., London 1995, @ £4.95 (ISBN 0 85934-385-5). There are a number of other similar booklets on the Maplin shelves all of which relate to interfacing the PC to various pieces of equipment. It is otherwise quite difficult to find books on the subject of interfacing at a suitable simple level, and even the above presupposes quite a lot of knowledge about electronics. We will explain what is necessary for you to understand what you are doing, and why.

The measurements are to be input into the PC via its Game (Joystick) connection (port).

Construction

Check that all of the components are present in your construction kit. There is a parts list in each kit. You will need to recognise what some of these are, but if in doubt ask a demonstrator. Keep components in their bags and wrapping paper until needed - no replacements!

Resistors - these are colour coded for resistance. (See separate code sheet)
Capacitors - labelled with the capacitance value.
Red LED - note the polarity, the small flat is on the negative pin side.
Opto-isolator IC - the darkened plastic represents an antistatic lining, don't touch the pins, leave it in its bag.
Operational amplifier (CA3140) - will be issued as necessary once the boards have been inspected.
DIL sockets - have a small depression at one end to indicate where pin 1 is placed when the socket is mounted on the pcb.
15 pin D-plug + metallised plastic cover - the pins are numbered on the blue plastic. We suggest that you solder this, and assemble it last of all even though you only need to solder two of the pins.
The pcb has been drilled ready to take the components. Don't finger the tracks. There are some markings for particular connections. You should look at the circuit diagram below and work out how the different components are placed on the pcb.
Two copper disks electrodes - don't peel the plastic off the copper until you need to for soldering.
Connecting cables and wires - supplied in the Tiger Lab as you need them.
Battery holder - needs four AA batteries, the twin press stud and lead to the pcb act as a switch to activate the stress meter. There is no separate switch as shown in the circuit.

PCB Layout
The diagram shows the view fromthe side of the pcb on which the components are mounted. The copper tracks are underneath. (Thanks to Nick Carter from the Chemistry Electronics Workshop for constructing these pcbs)
The Circuit
The circuit diagram for the device, corresponding to the above pcb layout :

Soldering
The old carpenter's adage 'measure twice cut once' becomes 'double check the component and its circuit position, solder once'. If neither you nor your partner has used a soldering iron before, then try practising your technique on some of the spare pieces of pcb, wires, etc in the boxes beside the soldering equipment.

Read the Antex Electronics Company's leaflet "How to Solder". (See separate copy)
Don't touch the pcb tracks with your fingers.
Place the component firmly in place before applying any solder.
Only cut excess wires from resistances, etc, once these are in place.
Try to use a minimum of solder for your joint, this will also prevent overheating components.
Keep the iron clean, deposits of flux and old solder build up on the 'bit'.
Inspect the solder joints for any 'solder bridges' between the pins or across the pcb tracks.

Connection cables
Once you have all of the components and DIL sockets mounted on the board, obtain the connecting wires for the D-plug, connect the plug, and then connect this to the pcb. The copper electrodes are connected across E1 and E2 on the pcb, before soldering the copper disks, press these over the metals rod provided in the Tiger lab, this will bend the disk into a shape which will attach to a finger when in use. Once bent, the disk can be soldered to the connecting wire.
Batteries
For the apparatus to work you will need to obtain 4 × AA batteries, these are not provided with the kits. (Tesco's or Boots prices are ~ £1.35 per 4-pack) However we will keep some batteries in the Tiger laboratory for test purposes.
Inspection
Once your stress meter has been constructed it must be checked by either Dr John Maher or Mr Ken Stewart before you proceed with Part 2. Please put a small label on the stress meter with your names, and an address or e-mail. Eventually, the stress meters will have to be handed in for marking, so when they are not in use, keep them safely in the component bag.