Building your own analogue sensors for the SmartBox

Friday, 29th November 2024

This is an extremely belated followup to a post from November last year, though one of the reasons for the delay was repeatedly being sent the wrong electronic components which ultimately led to the post from December. I'll try to explain as best I can...

Towards the end of last year I bought myself a SmartBox, a computer control system that's based around a 65C02 CPU and that runs programs written in an interpreted BASIC-like programming language, with software uploaded via a computer's serial connection (and once the programs are loaded onto the box, the serial connection can be severed and the program will continue to run). In the previous journal entry I focussed more on the software side, though to get practical use out of the box I had experimented with some of the hardware too and had built some analogue sensors that I was hoping to write up for the benefit of anyone else who'd acquired a SmartBox but none of the sensors to go with it.

Two home-made User Adaptors and two home-made temperature sensors.

The four analogue sensor ports on the side of the SmartBox use 5-pin DIN sockets (180° to differentiate them from the 240° 5-pin DIN socket used for the serial port). Each sensor port can measure an analogue value with an 8-bit resolution (between 0 and 255) with the maximum voltage refererence being set to 2.55V – that is, each unit in the 0–255 range reported by the sensor corresponds to 1/100V (10mV), so if you connected a 1.5V source to the sensor port you'd see a value of 150 returned.

Sensors were made that measured specific physical properties (such as light level, humidity, temperature, or sound level) as well as generic "User Adaptor" boxes that allowed users to connect their own sensors:

The SmartBox "User Adaptor" pictured in a brochure.

One other nice trick that the analogue sensors had was that they could report which type of sensor they were directly to the SmartBox, so when you plugged in a temperature probe (for example) it would know that it was measuring temperature on that sensor port. The way this was implemented is via a "sense signal" output pin from the SmartBox to the sensor. In normal operation it was low, and the voltage read on the analogue sense input pin was used as the sensor's current value. Once a second or so the SmartBox would drive this pin high, and the voltage read on the analogue sense input pin was used to determine the type of sensor.

There are 32 possible sensor IDs. The ADC reading mentioned in the table below is the minimum value that will cause the Smart Move software to identify a particular sensor, for example a humidity sensor can be identified with an ADC reading between 157 and 164:

Note that Smart Move 1.16 and Smart Move 1.18 have some different names for some sensors. If a space is left blank in the table then the sensor is treated as present but is labelled "SENSORA" to "SENSORD" instead of being renamed according to the sensor type.

An analogue multiplexer (such as a 4053 IC) can be used as the basis for an analogue sensor for the SmartBox, sending either the voltage to measure or a reference voltage set with a preset potentiometer to identify the sensor type. The "sense signal" output from the SmartBox chooses which of the two voltages to return:

When the adaptor is connected to the SmartBox the potentiometer RV1 can be adjusted to choose the sensor type. As this is only checked once per second it can be a little fiddly to dial in the specific value you want — temporarily bridging A0 and A1 on the 4053 (so that RV1 is used as both sensor type and input value) and watching the reported sensor value can make life easier (making sure any sensor is disconnected from the input socket, of course).

The internal components of the home-made User Adaptor.

Whilst this is a useful circuit for building custom sensors, or making a replica of the generic User Adaptor (my DIY effort pictured above) there is a simpler option that can be used for temperature sensors. The original temperature sensor doesn't seem to have very much circuitry of its own, being just a simple probe with a DIN socket on the end of a wire:

The SmartBox "Temperature Sensor" pictured in a brochure.

The temperature sensor provides a reading in Celsius directly in the software, e.g. a temperature of 25°C is reported with a value of 25. This corresponds to a 10mV/°C sensor, and an example of such a sensor is the LM35. One limitation of the LM35 is that without a negative supply voltage pulling down its output it reports a minimum temperature of 2°C, yet the brochure snippet shown above reports a range of 0°C to 100°C. However, reading the manual for the SmartBox it mentions that the Smart Sense temperature sensor measures temperatures between 2°C and 100°C which seems to further indicate that it is indeed something like the LM35.

We could therefore just connect an LM35 to the User Adaptor circuit shown above and that would work, however there is a simpler circuit. If you consult the table of sensor types at the start of this post you'll notice that the temperature sensor appears at the bottom of the table with the highest sensor value. This means that to be identified as a temperature sensor, the sensor must output at least 2.53V when the "sense" pin goes high. A crude way to implement this is to connect a diode from the sense output pin back to the analogue input pin:

When the sense pin is low the voltage at "input" will be higher than the voltage at "sense" so the diode will not conduct and not influence the output of the temperature sensor. When the sense pin is high, however, the voltage at "sense" will be higher than the voltage at "input" and so the diode will conduct and raise the voltage at "input" to above 2.53V, indicating that the connected sensor is a temperature sensor.

The internal components of the home-made temperature sensor.

According to the documentation for the SmartBox the inputs are protected against overvoltage so I believe this is a safe circuit to use, however as with the User Adaptor all circuits here are based on some rough guesses as to how the devices would have originally worked as I don't have any real sensors to test with myself. In any case, these sensors work for me as I've not been able to source any of the original parts, so maybe if you like me have an old SmartBox knocking around these may help you get some more use out of it.