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Yet Another Voltmeter

A project log for eForth for cheap STM8S gadgets

Turn cheap stuff from AliExpress into interactive development kits!

Thomas • 02/07/2017 at 22:44•7 Comments

After a day filled with business meetings | didn't want to risk killing the DC/DC converter for good, and I decided to look a little closer at one of the new arrivals from China, small and cheap voltmeter ($1.12 including shipping).

Repetitio non placet they say, but this voltmeter turned out to be really nice:

The LED display 3361BS is "0.36", red, common anode". The internal supply voltage is a bit unusual (3.0V), but otherwise the module is very hackable.

The 6 pads with 1.27mm pitch to the left are connected to the following signals: 

1 SWIM (7S-LED segment D)
2 TxD
3 RxD
4 NRST
5 +Ub
6 GND
The STM8S003F3P6 pins are assigned as follows:
STM8S003F3P6assigned toSTM8S003F3P6assigned to
1 PD47S-6 (NC!)20 PD37S-5 G
2 PD5Pad2 TxD19 PD27S-4 C
3 PD6Pad3 RxD18 PD1/SWIM7S-3 DP, Pad1 SWIM
4 NRSTPad4 NRST17 PC77S-2 D
5 PA1
7S-9 Digit 216 PC67S-1 E
6 PA2(7S-9 Digit 2)15 PC57S-7 B
7 VssGND14 PC4/Ain2TP (535k-Ain-8k26-GND)
8 VcapC13 PC37S-12 Digit 1
9 Vdd+3.0V12 PB47S-10 F
10 PA37S-8 Digit 311 PB57S-11 A

I guess that the designer was bored by making yet another cheap Chinese voltmeter, and he decided to make a hackable voltmeter instead. If not, why would one expose not only SWIM/NRST but also RxD and TxD? Why route a spare GPIO to the (unpopulated) NC pad of the LED display?

Hat-tip to China!

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Discussions

Thomas wrote 02/13/2017 at 18:11 • point

Anders, right, that's the same type of voltmeter. The price is very good.

  Are you sure? yes | no

davedarko wrote 02/07/2017 at 23:48 • point

Hm, they too have to program it somehow, so a port like this would make sense, if they don't get those chips pre-programmed, right? But good to know there are STM8s under there (probably) :) 

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Thomas wrote 02/08/2017 at 06:28 • point

Some of the boards seem to be produced with pre-programmed controllers. Some "DC/DC converter with voltmeter" boards are in this category. 

On the other hand, access to GND and to PD1/SWIM is sufficient under certain (easily met) conditions for ICP. There is a family of "relay boards" that might be interesting, but PD1/SWIM appears to be unused, and that's why I didn't buy one for this little project.

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K.C. Lee wrote 02/07/2017 at 22:55 • point

Just make sure it is not this one as it has a different microcontroller with 14 pins with no spares.

https://hackaday.io/project/7700-collection-of-misc-small-project/log/30822-cheap-voltmeter-from-china

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Thomas wrote 02/07/2017 at 23:01 • point

You're right, you can never be sure what you get, but in this case pads labeled RxD and TxD were easy to spot.

I ordered here: 

https://www.aliexpress.com/item/New-LED-Digital-Display-Voltmeter-Panel-DC0-200V-0-36Inch-Direct-Current-Red-high-quality-free/32565027508.html

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