{"id":635,"date":"2026-05-25T00:46:13","date_gmt":"2026-05-24T16:46:13","guid":{"rendered":"https:\/\/darcyjprojects.xyz\/?p=635"},"modified":"2026-05-28T14:34:23","modified_gmt":"2026-05-28T06:34:23","slug":"designing-a-4-20-ma-transmitter","status":"publish","type":"post","link":"https:\/\/darcyjprojects.xyz\/index.php\/2026\/05\/25\/designing-a-4-20-ma-transmitter\/","title":{"rendered":"Designing a 4-20 mA Transmitter"},"content":{"rendered":"
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Figure 1. <\/strong>Assembled Current Transmitter with Cover and Moisture Sensor<\/figcaption><\/figure>\n<\/div>\n\n\n

For my Industrial Embedded Automation project<\/a>, I needed a way to test the 4-20 mA input functionality, however all my test sensors were simple 0 to 3.3 V voltage sources. To bridge the gap, I designed a custom voltage to current transmitter sub-board utilising an op-amp and transistor to push current through the loop.<\/p>\n\n\n\n

Initially, I considered building this as a purely analogue circuit to keep things simple. However, the moisture sensor I was testing was a very basic “dumb” sensor that didn’t output linearly over the full 0 – 3.3 V range. Tuning an analogue circuit to scale that specific, narrow voltage band up to a full 4-20 mA swing would have required adding manual trimmer potentiometers and a lot of hardware tweaking.<\/p>\n\n\n\n

Instead, I leaned into the “overkill” approach and added an ATMega328P microcontroller. There would have been many better options, but I only had STM32s on hand to prototype with that would have been 10x more overkill!<\/p>\n\n\n\n

While even the 328P might seem excessive for a simple transmitter, it gave me two massive benefits. First, I can handle the sensor calibration entirely in software, mapping the exact sensor limits to the 4-20 mA output perfectly. Second, I can oversample the analogue voltage readings and average them out to drastically reduce random signal noise before converting it to a current output.<\/p>\n\n\n\n

To make debugging and calibration easy, I included the ubiquitous CH340G USB to Serial IC – all too well known by its prominence on Chinese Arduino clones. It keeps the BOM cost incredibly low while allowing me to monitor the scaled values over a standard serial monitor.<\/p>\n\n\n\n

I designed this board to take a 5V input directly to power the 328P, with a simple AMS1117 linear 3.3V regulator to power the sensor. <\/p>\n\n\n

\n
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Figure 2. <\/strong>Current transmitter schematic (0-3.3 V sensor to 4-20 mA current loop)<\/figcaption><\/figure>\n<\/div>","protected":false},"excerpt":{"rendered":"

For my Industrial Embedded Automation project, I needed a way to test the 4-20 mA input functionality, however all my test sensors were simple 0 to 3.3 V voltage sources. To bridge the gap, I designed a custom voltage to current transmitter sub-board utilising an op-amp and transistor to push current through the loop. Initially, […]<\/p>\n","protected":false},"author":1,"featured_media":654,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[20],"tags":[14,9,37,13,17,10],"class_list":["post-635","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-electronics","tag-analogue","tag-electronics","tag-industrial","tag-microcontrollers","tag-op-amps","tag-pcb-design"],"featured_image_src":"https:\/\/darcyjprojects.xyz\/wp-content\/uploads\/2026\/05\/current_transmitter_thumbnail2.png","author_info":{"display_name":"Darcy","author_link":"https:\/\/darcyjprojects.xyz\/index.php\/author\/darcy\/"},"yoast_head":"\nDesigning a 4-20 mA Transmitter - DarcyJProjects<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/darcyjprojects.xyz\/index.php\/2026\/05\/25\/designing-a-4-20-ma-transmitter\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Designing a 4-20 mA Transmitter - DarcyJProjects\" \/>\n<meta property=\"og:description\" content=\"For my Industrial Embedded Automation project, I needed a way to test the 4-20 mA input functionality, however all my test sensors were simple 0 to 3.3 V voltage sources. To bridge the gap, I designed a custom voltage to current transmitter sub-board utilising an op-amp and transistor to push current through the loop. 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