Description

Remote Alarm transmitter

As part of my experiments with LoRa and the easy to use ATMega328P, I have recently designed quite a few LoRa based projects. In this final 2 part series, I will look at two additional projects, part of a Remote Alarm Transceiver, where I experimented with a changing a few things:

– Using LM317G adjustable voltage regulators.

– Replacing my standard N-MOS based logic level converters with a dedicated chip.

LoRa Remote Alarm Transmitter – with Onboard Relay putout and two sensor inputs

How does this differ from my other LoRa Based projects?

The PCB presented above does in fact not really differ a lot from any of my existing LoRa based projects.

However, there are a few subtle changes, mainly experimental changes, brought on by factors such as component availability and an attempt to reduce component counts and board size.

The first of these changes is using the LM317G voltage regulator, in the place of my usual LM1117 3.3 and 5.0 LDO regulators.

The LM317 is an old device, It has been on the market for a long time. It can supply up to 1.5A of current, and a single device can be configured to supply a wide range of different voltages by just changing two resistors. This seemed quite attractive to me, as it is getting quite difficult to reliably get quite a few components on time, and with decent pricing in the post-Covid-19 world.

The second major change would be moving away from my existing N-Mos based Logic converter setup, where I used the BSS138 and 10K resistors as logic converters. This setup works perfectly, but it has the drawback of requiring quite a lot of components. for example:

To provide logic conversion to an RA-02 module, with access to all the IO Lines (GPIO0-5 included) required 12 BSS138 Mosfets and 24 10k resistors. This is quite a lot of components. A dedicated logic converter chip would thus be a much more attractive solution.

Driver circuitry for sensor Inputs, consisting of a simple transistor input, and an optically isolated Relay output completes the circuit.

Using the LM317

The output voltage of the LM317 is typically set using two resistors, with a suitable current rating, using the following Formula

VOUT = 1.25 * ( 1 + R2/R1 )

It is also common to use a variable resistor at R2, to have fine control over the output voltage. This is due to the fact that stock resistor values do not always give you the exact voltage you require. You should also take into account that using a 5% resistor will be less accurate than a 1% resistor.

Another issue will definitely be heat dissipation. In my PCB design, I have used the SOT-223 package of the component, with a PCB heatsink, built directly into the layers. With the LM1117 LDO regulators, these work extremely well.

Logic Level Conversion

In this design, I used my standard Logic Level conversion circuit, comprised of a BSS138 N-Mos with two 10 k resistors. This circuit, although a bit cumbersome with lots of components if you need many logic converters, is very stable, and functions extremely well.

Conclusion

This circuit was designed as a two-part prototype, with the goal of experimenting with different voltage regulators, and in part 2, with a single chip 8 channel logic converter. As such, I do not feel comfortable releasing the full schematics to you at this stage, do so anyway in the interest of learning. The circuit works, but there are many issues with the regulators: