Diode D1 has its positive leg lifted away from the PCB.
An external 2.5mm power connector is attached to the diode and to a ground pin underneath the PCB.
Temperature
A low-cost temperature logger with a serial-data
output is available from a supplier in the UK here:
www site.
It uses a PIC-chip and monitors temperature on
4 channels. The hardware is a commercial product, the design and software
is now open-source. The designer lives in Australia and provides detailed
notes and source-code here.
Data readings are transmitted at 2400-baud speed
on pin 2 of a 9-Way D-Type female serial-connector.
This DCE device is designed without a
power source. It expects to find a positive voltage on an incoming DTR
signal, on pin 4, output by a host device attached to the logger. But the
5-volt DTR voltage provided by a Newton PDA is insufficient to power the
logger, so a modification is required. An external power-connector is easily
attached to this circuit.
Diode D1 has its positive leg lifted away from the PCB.
An external 2.5mm power connector is attached
to the diode and to a ground pin underneath the PCB.
Two 9-volt PP3 rechargeable batteries wired in parallel provide plenty
of power for the temperature logger. For prolonged use, a 9-volt DC mains-adapter
can be used instead of batteries.
One temperature sensor is shown attached to the logger. Longer cables (~10-metres) attached to the sensor will work fine.
A Newton running fleXYlog software can
process the data from the temperature logger.
This table-top experiment shows the logger in
use:
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The picture from a Newton 110 shows the change in temperature when a teaspoon of salt is added to a glass of water. The temperature is rising slowly from 14.8 C to 15.25 C over 10 minutes. Salt is added and stirred, and the temperature quickly drops to 15 C within 30 seconds. When the water is saturated with salt, the temperature ceases to fall.
If iced water is used, the temperature will drop
below freezing point and will remain liquid after the salt is added.
(This is a useful way to generate negative temperatures,
for testing PIC code.)
The temperature sensor supplied with the logger
needs to be protected from damage.
Here it is encased in a small aluminium tube
(obtained from a hobby-shop) and painted with several coats of lacquer.
The aluminium tube is sealed with tape or heat-shrink plastic.
The DCE output voltage-levels from the logger are 0-5 volt TTL, and a modification is needed to allow connection with a Newton PDA having an RS-422 interface with a standard DCE PC cable.
Simple gender-changer to convert DCE to DTE pinout.
Pin 2 (Tx data) on the logger DCE connector is
routed to pin 3 (Tx data) on the outer DTE connecter.
Pin 5 is joined to pin 5 (ground) on each connector,
using solid copper wire from some cheap 75-ohm TV aerial.
This allows a standard Newton DCE PC serial cable to be used - except for one snag !
My 2-metre Apple-branded serial cable (part # 590-0964) refuses to work.
serial-cable pinout is here: cable
details.
The problem seems to be related to pin-8 of the 8-pin DIN connector and the RS-422 interface.
Experiment shows that pin-8 (Rx+) needs to be disconnected from ground at the 9-Way D-Type end and either left unconnected, or re-connected via a 47nF capacitor to ground (ground is pin 5 on the 9-Way D-Type connector).
As the Apple cable is moulded and sealed, it it easier to make a new cable.
Capacitor connected between black wire and ground pin-5.
Black wire leads to pin-8 on the 8-pin DIN end.
See the cable-pinout for these
modified wiring details.