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This project has been created to monitor a Raspberry Pi to make sure that it is healthy and running as it should be. Although it is intended to be part of our Nexus DR-X Digital Radio Cross Patch stack, the Nexus Watchdog can be used with any device that is capable of putting out a digital pulse that we call the "heartbeat".
Our watchdog sits in line between your device and its power supply. When all is well, the power is simply passed along and your guy runs as normal, executing what ever code it needs as well as a simple script or sketch that sends a 3.3 or 5 volt pulse out a GPIO and into the watchdog once every few seconds.
The kit is priced at $36USD and includes all through-hole parts. The PCB footprint matches a standard Raspberry Pi 3 or higher. We even through in standoffs so that you can mount it either above or below your Nexus DR-X stack. The Nexus Watchdog doesn't care what you are asking it to monitor. It can be an Arduino or an ESP32; literally any microcontroller or microprocessor that is cable of outputting a GPIO heartbeat.
The Nexus Watchdog is 100% solid state hardware driven. There are no 1's or 0's here to worry about bits flipping. It's all old school basic electronics. We have two RC timer circuits running a CD14538B monostable multivibrator chip and a mechanical relay to turn things on and off.
The first timer is our "Proof of Life" timer. A slider switch allows you to choose a short window of 4.7 seconds for test purposes or a longer one of about 30 seconds for reliable unattended monitoring of your device. Every time the heartbeat is detected, the Proof of Life timer is reset. As long as that pulse continues your equipment will continue to operate for days, weeks, even years.
If something goes wrong and things lock up causing the heartbeat to stop, the Proof of Life timer will run out. When that happens, a relay is activated which pulls the plug on your programmed device and starts the "Power Off" timer. 10 seconds later the second timer runs out and the power is turned back on. The watchdog provides a full power down and then restarts what ever it is that you need to monitor.
The timing intervals are determined by the values of a resistor/capacitor pair. These are classic RC circuits, proven and reliable. The formula is simply T=RC. To make the math as easy as possible, we are using 100 μF electrolytic capacitors. When paired with a 330K Ω resistor, the math works out to 33 seconds. The short window, for testing, uses a 47K Ω resistor to give us a 4.7 second window. Think of it this way: nK ohms = n/10 seconds.
The Nexus Watchdog uses a 12 V relay to turn off your equipment. You simply pass the Vcc through the watchdog using either a standard DCV barrel jack or a pair of screw terminals. The power is passed through the relay to a second pair of terminals. You hook your regular power supply up to the watchdog and make a short cord to run to your device.
The CD14538B will run on anything from 3 to 20 volts. The data sheet recommends values between 5 and 15 volts. Please note, however, that if you plan to power this with 5 V you will need to switch out the 12 V relay with a 5 V relay. They are available and will fit in the same holes. The 12 V relay will work fine at 6, 9, 12 and 13.8 volts.
Our watchdog sits in line between your device and its power supply. When all is well, the power is simply passed along and your guy runs as normal, executing what ever code it needs as well as a simple script or sketch that sends a 3.3 or 5 volt pulse out a GPIO and into the watchdog once every few seconds.
The kit is priced at $36USD and includes all through-hole parts. The PCB footprint matches a standard Raspberry Pi 3 or higher. We even through in standoffs so that you can mount it either above or below your Nexus DR-X stack. The Nexus Watchdog doesn't care what you are asking it to monitor. It can be an Arduino or an ESP32; literally any microcontroller or microprocessor that is cable of outputting a GPIO heartbeat.
The Nexus Watchdog is 100% solid state hardware driven. There are no 1's or 0's here to worry about bits flipping. It's all old school basic electronics. We have two RC timer circuits running a CD14538B monostable multivibrator chip and a mechanical relay to turn things on and off.
The first timer is our "Proof of Life" timer. A slider switch allows you to choose a short window of 4.7 seconds for test purposes or a longer one of about 30 seconds for reliable unattended monitoring of your device. Every time the heartbeat is detected, the Proof of Life timer is reset. As long as that pulse continues your equipment will continue to operate for days, weeks, even years.
If something goes wrong and things lock up causing the heartbeat to stop, the Proof of Life timer will run out. When that happens, a relay is activated which pulls the plug on your programmed device and starts the "Power Off" timer. 10 seconds later the second timer runs out and the power is turned back on. The watchdog provides a full power down and then restarts what ever it is that you need to monitor.
The timing intervals are determined by the values of a resistor/capacitor pair. These are classic RC circuits, proven and reliable. The formula is simply T=RC. To make the math as easy as possible, we are using 100 μF electrolytic capacitors. When paired with a 330K Ω resistor, the math works out to 33 seconds. The short window, for testing, uses a 47K Ω resistor to give us a 4.7 second window. Think of it this way: nK ohms = n/10 seconds.
The Nexus Watchdog uses a 12 V relay to turn off your equipment. You simply pass the Vcc through the watchdog using either a standard DCV barrel jack or a pair of screw terminals. The power is passed through the relay to a second pair of terminals. You hook your regular power supply up to the watchdog and make a short cord to run to your device.
The CD14538B will run on anything from 3 to 20 volts. The data sheet recommends values between 5 and 15 volts. Please note, however, that if you plan to power this with 5 V you will need to switch out the 12 V relay with a 5 V relay. They are available and will fit in the same holes. The 12 V relay will work fine at 6, 9, 12 and 13.8 volts.
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This video is a segment taken from The Mount Baker Amateur Radio Club's Digital Group Zoom Meeting where I show all the parts that go into the Nexus Watchdog kit and talk about what many of them do in the circuit.
You will find a slideshow of these step-by-step photos in the menu above. This video should give you a good idea of what it will be like to build your own kit. |
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