IR dongle pcb
Initially I have built a circuit on a breadboard with a phototransistor at the end of a long wire connected to an Arduino UNO. It worked great for a while, but then I decided to build a more permanent circuit on a self made PCB. The Arduino was replaced with a FT232RL USB to UART bridge (datasheet). The final assembly on top of the Smartmeter looks like this:
I designed the circuit and board using the popular EAGLE sofware from Autodesk. The IR signal from the energy meter is received with an Osram SFH309 phototransistor and converted to digital TTL level using a LM324 operation amplifier. The op amp circuit is working as a voltage comparator giving non-inverting Schmitt trigger output. The TX signal is connected to the FT232RL, which transmits the signal to the USB host. Initially I used the MCP2221 I2C/UART USB bridge from Microchip (in DIP package) until I found out that it doesn't support the 7-bit data mode required by the OD-type Smartmeter.
The final board layout looks like this:
Bill of materials with order numbers from Reichelt Elektronik required to build the circuit:
| Quantity | Type | Value | Package | Order No. |
|---|---|---|---|---|
| 1 | FT232RL USB to UART | SSOP-28 | FT 232 RL | |
| 1 | LM324 Operational amplifier | DIP-14 | LM 324 N | |
| 1 | IC socket | 14 pin | DIP-14 | GS 14P |
| 1 | Pin header | 40 pin | RM2.54 | BKL 10120206 |
| 1 | USB panel socket | USB-B | USB BW | |
| 1 | Potentiometer | 10K | 10mm | PT 10-L 10K |
| 1 | Phototransistor | 730...1120nm | THT-3mm | SFH 309 FA |
| 1 | LED green | 20 mcd | THT-5mm | KBT L-7113GD |
| 1 | Transistor NPN | 0.8A | TO-92 | BC 337-25 |
| 1 | EMC ferrite filter | 90R | BEAD 4-90 | |
| 1 | Film Capacitor | 10nF | RM5.0 | MMK 10N 250 |
| 3 | Ceramic capacitor | 100nF | RM5.0 | KERKO 100N |
| 1 | Ceramic capacitor | 10nF | RM2.5 | KERKO 10N |
| 1 | Capacitor | 4.7 µF | RM2.5 | M-A 4,7U 50-2 |
| 2 | Capacitor | 1 µF | RM2.5 | M-A 1,0U 250 |
| 1 | Resistor | 2M | 0207 | METALL 2,00M |
| 1 | Resistor | 100K | 0207 | METALL 100K |
| 3 | Resistor | 13K | 0207 | METALL 13,0K |
| 1 | Resistor | 10K | 0207 | METALL 10,0K |
| 1 | Resistor | 270R | 0207 | METALL 270 |
| 1 | ABS case enclosure | 129x40x26 mm | SP 2090 SW | |
| 1 | Photo PCB, single-sided | 160x100 mm | BEL 160X100-1 |
The CAM output from the EAGLE CAM processor was loaded into Gerbv. Gerbv allows to overlay different CAM layers making up the final layer mask. Specifically, you need to overlay the profile, drill and bottom copper layers and choose black, white and black colors, respectively. Make sure to disable the "Show cross on drill holes" option in the View menu. The layer mask was printed onto transparent paper.
Further Instructions for processing the PCB can be found here. To make the PCB, the following steps were done:
- The printed layer mask was laid flat onto the glass of a ProMa UV Belichter 1 with the printed side pointing towards the PCB
- The positive photo PCB (160x100mm) was exposed to UV light for 45 seconds.
- The exposed PCB was developed using NaOH solution for 2:30 minutes followed by etching in NaPS solution for about 30 min.
After cutting, sanding and drilling (with a battery powered screwdriver) the components were soldered onto the PCB. A good tutorial for soldering the FT232RL SMD chip I found on youtube. The SMD pads were thoroughly cleaned with isopropyl alcohol and pre-tinned before soldering as shown in the video. The dimensions for the case holes and cutouts are given in the Smartmeter manual.
In order to enable the on-board LED, the programmable outputs C0 and C1 need to be swapped (C0->RXLED#, C1->RXLED#) using the FT_PROG application as shown in the screenshot:
