Teardown Tuesday: General Bluetooth Multimeter

'General Tools' distributes a variety of measurement tools through home improvement stores and online resellers. Their latest 'Toolsmart' line of tools includes Bluetooth functionality. This week, AAC purchased an auto-ranging Bluetooth multimeter to take apart and peek inside.

About the Multimeter

The Toolsmart line of products by General Tools provides Bluetooth connectivity to several different measurement products that can connect to iPhone and Android smartphones through the manufacturer's provided apps (via the Google Play and iTunes Store). The multimeter used in this teardown is of comparable quality to others in this price class (<$40) with the exception that this one has Bluetooth connectivity. Once connected, data is streamed from the multimeter to an app on the user's phone that can then record and save the data in lists or on top of photographs taken by the smartphone.
Limited testing of the device prior to the teardown showed an easy-to-use app that connected immediately to the multimeter with an iPhone.

The multimeter (intact). Image courtesy of Amazon.

Taking Apart the Multimeter

Disassembly of the multimeter is relatively straightforward. Disconnect and set aside all probes before you begin.
To start, remove the battery case screw and the battery cover. Then remove the soft plastic shell that surrounds and protects the hard plastic case from small bumps and scrapes. The bottom side of the multimeter holds four thermoplastic case screws that hold the two sides of the multimeter together.
For most users, disassembly stops here, as it provides access to the fuses. For others, continue the teardown by removing all visible thermoplastic screws holding the circuit board to the front of the case.
Remove the circuit board from the front half of the case by either pushing the banana plugs through from the front or by patiently and gently pulling on the circuit board to free them from the circuit board side.
See the video below for more information.

A Look Inside

Image shows the reverse of the top-side of the case, the front of the multimeter circuit board and the back of the multimeter circuit board.

Top Side Marking	Description	Cost	More Information
DTM0660L	Multimeter IC		Datasheet (Chinese) | Datasheet (English)
24C02A	EEPROM	$0.1	Datasheet | Alternate Datasheet Link
BDE-BLEM201P	Bluetooth UART		Datasheet

The center image highlights the copper rings (4) that create the rotary selection switch. The right image highlights the important ICs on the circuit board (1 through 3).

(1)  DTM0660L (Pink)

DreamTech International, a Hong Kong-based company manufactures the DTM0660 integrated circuit. It is a very popular multimeter microcontroller that performs all of the analog-to-digital (ADC) conversion and drives the LCD display. Configuration data is stored in an external EEPROM.
The datasheet for the DTM0660 is written in Chinese. Fortunately, a bilingual engineer named Kerry Wong translated the datasheet, including the bitmap, into English. If you need to modify a DTM0660-based multimeter, his site appears to be a great place to start as it provides information on reading and writing the I²C based EEPROM.

   

 

(2)  T24CO2A (Yellow)

This is a two-kilobyte I²C EEPROM that stores the configuration information for the main microcontroller. It accompanies the DTM0660 multimeter IC so frequently that its presence allows hobbyists and hackers to surmise the presence of the DTM0660 circuitry beneath the epoxy of inexpensive chip-on-board multimeter designs.

Schematic used courtesy of Atmel via Mouser.

(3)  BDE-BLEM201P (Blue)

Image courtesy of BDE

 

This decidedly small module integrates a Bluetooth Low Energy (BLE) radio, the Bluetooth software stack, and an 8051 microcontroller.  It has 23 General Purpose Input/Output pins, 8K Ram, 128K flash memory, and supports one I²C and two USART interfaces.

Image courtesy of BDE

 

These modules seem to be purpose built for painless pairing with cell phones, and a datasheet for a different product that the company sells also bore pictures that resembled the 'General Tools' app.  Unfortunately, the datasheet was written entirely in Chinese, and the email I sent to the company has not been returned.  My suspicion is that the company facilitates app development by providing code and structure for iPhone and Android apps.

(4) Rotary Selection Switch (Orange)

Specialty switches can be expensive and, in certain instances, electrical engineers can save a great deal of money by designing their own out of copper traces and copper spring contacts.
As I mentioned in the previous teardown of a different device, users need to know that the switch is in position, and solutions can be inexpensively engineered by including cantilevered cams with corresponding indentations. This solution works for linear and rotary switches.
The mechanical engineer of this device used four cams and indentations on the back side of the multimeter knob to simultaneously provide physical feedback for users and to positively lock the dial in position.

 

Conclusion

I would like to again point out that AAC purchased this multimeter for this teardown and that we have no ties to General Tools.
Recently, I have been tearing down quite a few Bluetooth tools, and am usually so disappointed by their lack of functionality that I do not bother to reassemble them. I put this multimeter back together immediately after the teardown.

If you need a Bluetooth-connected multimeter and have a few dollars to spend ($33.50 at Home Depot at the time of publication), this is certainly worth consideration.