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New: The Thunderbolt Monitor Kit is now available!

The Thunderbolt Monitor Kit

Now with new firmware correcting the GPS Week Rollover issue

The kits are available again, to purchase, see Paypal buttons below

This page describes a low cost Thunderbolt monitor kit based on the original, widely deployed and shamelessly copied Thunderbolt Monitor described on this site.

Picture 1: the kit

Note: after July 30th 2017, the Thunderbolt GPS receiver will report the wrong date. It is not a Thunderbolt problem per se, even though it could have been handled better, it is directly related to an issue with the GPS system. Please see this page for information about how to deal with it.

Compared with the previous kit, the PWB has been modified to support additional functionality. There are more IOs available on headers, including analog inputs that can be used for temperature monitoring with cheap thermistors, and outputs to directly interface with inexpensive relay modules from eBay for features like the alarms. For applications where the Thunderbolt Monitor kit cannot be located close to the Thunderbolt receiver itself, an optional XBee transceiver could be used to wirelessly carry the TSIP data between the two.

The voltage regulators are now heat sunk to large PWB planes, so the kit can be operated from 12V safely even with the WiFi module over long periods. The maximum supply voltage is still 16V (due to the voltage rating of the regulators and decoupling capacitors) and it is not recommended to operate the module over long periods with more than 12V to ensure proper derating.

There are 4 LEDs on the board. Two are used to indicate minor and major alarms respectively. Another is used to indicate when the optional WiFi module is linked to your router, the last one indicates when the TB Monitor is connected through the WiFi module (and an IP address is available.) 

The pulse stretcher, useful for those wanting to use NTP is still there, now able to work from higher impedance sources (the original kit only worked with the very low output impedance of the Thunderbolt's PPS output).

The PWB size is 3.8" x 2.5".

I also offer a WiFi module as an option for $25, or about half the cost of the WiFly used on the previous kit. This is not the common ESP8266 module which has too many limitations in its current iteration. The module I selected has a full TCP/IP stack and supports SoftAP setup.

By using a larger (and cheaper) screw terminal strip, the price for the basic module (no WiFi) remains at $80 + shipping, in spite of the additional functionality. Shipping is $10.00 for delivery via USPS with insurance anywhere in the continental United States, $25.00 without insurance for delivery to Canada. 

International shipping:

Contact me via email (see at the bottom of this page for link) if you live abroad. The USPS is very inconsistent with their prices, so I will quote each request based on specific quote from the USPS for your location. For example, shipping to Germany and Finland is $34.00, to Australia it is $35.00 .I anticipate shipping to most countries will be in that range, but with the US postal service, I prefer to check in every case. The USPS says delivery to most major markets is 6-10 business days. That has been my experience, even though if you are unfortunate to live outside of a major market, it could take much longer. Keep in mind the USPS is only responsible for the package until it gets to your country's customs department. For the remainder, your country's customs and postal service are responsible.


As of October 2017, I will try using the USPS' First Class Mail service instead of Priority Mail. The cost is about $15 instead of $35 for most countries and deliveries is quoted as 2-3 weeks for most destinations. Still, contact me for a quote for your country.

To order:

TBMon ship to USA
($9.45 USD shipping)
TBMon ship to Canada
($24.95 USD shipping)


If you live outside the USA or Canada, send me an email (email link at the bottom of page) indicating your country and I will send you a shipping quote.

Main Features:

  • Kit is delivered complete and ready to go. Packaging is up to you.
  • No soldering required for setup or operation. Connections to Thunderbolt and power supply are through screw terminals.
  • You provide the power supply, cables and connectors.
  • Kit includes a ~1mS "PPS stretcher" for NTP or other applications not able to deal with the narrow Thunderbolt PPS pulse (10μS).
  • Kit includes capability to send a self survey command without additional hardware or computer.
  • Kit can be ordered with an optional WiFi module to provide TCP/IP access to your Thunderbolt via your wireless network. The WiFi module can be ordered later separately. If so, shipping charges will apply.
  • Alarms and time mode (GPS, UTC, local, time zone) are user configurable via menu.
  • Expansion capability through GPIO, I2C interfaces and an XBee socket for this or other applications.

Things you need to do:

  • Provide and connect a 7 to 12 VDC supply rated 250mA minimum (400mA if you install the WiFi module). Alternately, the 5V regulator can be bypassed if you wish to power the module from a regulated 5V (such as USB charger)
  • Provide and connect a DB-9P connector for connection to the Thunderbolt.
  • The kit is provided without an enclosure. Therefore, it is more static sensitive than regular consumer equipment. Follow safe handling practices to prevent damage (see this page.) This is particularly true in dry climates and during the winter everywhere in the northern hemisphere (heating dries the air in your house.)

WiFi option:

The WiFi option allows the Thunderbolt Kit to become a server compatible with the Lady Heather Thunderbolt Monitor software. It consists of a small PWB that is soldered on the back of the main PWB.

Picture 2: The WiFi module on the back of the main board

The WiFi module comes with a coaxial connector (type U.fl) and a small antenna that can be installed inside or outside a plastic enclosure (Figure 2a). If you want to mount the kit inside a metal enclosure, I recommend using a antenna better suited like the one shown in figure 2b.

Picture 2a: The WiFi module with its antenna

Figure 2b: a WiFi antenna suitable for metal enclosures

Picture 3: LadyHeather monitoring my Thunderbolt

The WiFi option has otherwise no effect on the kit's operation, which can still be used to monitor the Thunderbolt via its LCD and menu while the server is running.

The WiFi module can be operated with a fixed (static) IP address or with an IP address obtained via DHCP (the default for most home networks).

The WiFi module comes with a small PWB antenna and a ~4-6" coax cable which plugs into the WiFi module with a U.FL connector. The antenna can be easily replaced with a chassis mounted dipole of the type widely available on eBay and other places which allows to mount the TBolt kit inside a metal enclosure while keeping the WiFi functionality.

Aside from the method to obtain an IP address, the WiFi module has to be configured with the SSID and passphrase setup in your WiFi access point or router. The WiFi module is configured through the SoftAP feature. You only need a PC or tablet with WiFi to configure the WiFi module. The manual will have instructions to show you how to do that.

User's Manual:

The User's Manual for firmware version 4.2.2 is available here. There is a separate section for the WiFi module setup. Note that there are two similar WiFi modules, each has its own manual since the setup is different (but operation is the same). The original manual is here, the new manual is here

For users of the previous generation kit, a copy of the manual version 3.5.0 is available here.

Design details:

The processor used is the Silabs C8051F587-IQ. It has 96kB of flash memory, 8kB of RAM, two serial ports and lots of other goodies. This processor will allow much more powerful applications than the original TBolt Monitor. For reference, the original TBolt Monitor used a processor with 8kB of flash, and only ~25% was used.

The kit supports a serial EEPROM used to store time mode (GPS, UTC or Local), time zone and other settings.

See Notes at the bottom for more information.


The schematic is shown in Picture 4. 

Picture4: Schematic of the Thunderbolt Monitor kit

PPS Extender:

Here are waveforms from the PPS extender:

Picture 5: The PPS from the Thunderbolt is the yellow trace (the pulse is about 10μS wide, it would not be visible on an analog scope at that time scale),
The blue trace is the voltage across the capacitor.
The purple trace is the output from the first stage of the MAX3232.
The actual pulse from the production kit is a little narrower, around 1mS, still more than enough for NTP.


Picture 6: The leading edge detail showing the ~500nS delay between Thunderbolt PPS output (yellow trace) and RS-232 output (green trace). Note that faster response is not necessary since this pulse will drive an RS-232 receiver in the computer, with similar characteristics.

Please note that only the leading edge of the output of the PPS Extender should be used for timing application as the pulse width is not well controlled (temperature dependant).

The pulse extender circuit is protected against PPS voltages as high as Vin + 5V and as low as ground. Exceeding these voltages may damage Q1.

Alternative displays:

The kit will ship with a 2x16 blue display as shown in Picture 1.

The interface is the standard Hitachi HD44780, which means many other types of displays can be used, including larger LCDs or Vacuum Fluorescent Displays (VFDs) like the one shown in Picture 7.


Picture 7: VFD Display, Manufacturer: Noritake, Part Number: CU16025-UW6J

The Noritake part shown in Picture 7 above is an almost exact fit for the stock LCD display, only the mounting holes are just a little too small for the (metric) mounting hardware coming with the kit, so you may have to file the holes on the display or use different hardware.

However, not all HD44780 compatible displays have compatible dimensions or the physical layout of the connector may vary. Therefore if using a different display, you may have to wire it to the board instead of simply plugging it in.

A compatible header type is the Samtec TSW series, like the TSW-150-07-T-S (available from Digikey and a number of other distributors.)

Finally, power requirements may vary. See the User's Manual for further information.

Relay Modules for Alarm Outputs:

To drive high current loads from the alarm outputs, I recommend using inexpensive relay modules like those shown in Picture 8, available on eBay. Simply search eBay for "relay module"

Picture 8: an inexpensive relay module



  • The most useful information to assess the Thunderbolt's operation rests in the alarm messages. If there is no alarm, the Thunderbolt operates under nominal specified conditions, and that is the best we can tell without external instrumentation.
  • The PPS extender is designed to work with the Thunderbolt receiver which provides a 0-5V, 10μS pulse. Other GPS receivers may have a different PPS pulse and this circuit may or may not work.
  • You can use my Thunderbolt Simulator software to verify operation of this or other Thunderbolt monitoring device.
  • Check the Lady Heather software for Thunderbolt monitoring and control, by John Miles/Mark Sims and other contributors.