Weight/Load Sensor Recommendations - Page 1
Forum section: electronics / projects

author: remixed123
author_rank: Regular Contributor
timestamp: May 16, 2015, 05:12:57 pm
content: I have been trying to find a weight sensor that can measure from 0 - 3KG (6lbs). My preference is one that has I2C, so I can place it on the bus with the other sensors. If there are no I2C one, then one that can work with the ADC input on my MCU.My whole solution will run on batteries, hence I need to reduce power consumption as much as possible.Any experience of opinions on a device that fits my requirements, and also any advice on what the best practice is for reducing power consumption with these sensors.

author: Paul Price
author_rank: Super Contributor
timestamp: May 16, 2015, 11:54:27 pm
content: Use a cheap 8 to 28 pin PICC or ATMEL MCU with I2C and then use an external 8.0000MHz xtal for a reference frequency.Set the internal clock to 8 MHz and the frequency will change depending on the pressure placed on top of the MCU plastic case.Program the MCU internal coutner-timer to measure the freq. (period) of the external xtal oscillator period connected to an IRQ pin. The MCU measured period (freq.) of the external xtal osc will change with pressure applied to the case of the MCU.If you configure the MCU to go to sleep and awaken upon I2C interrupt the power used can be very low.You will have to calibrate your MCU (A.K.A. weight sensor) with known weights and possibly the MCU may need to take the ambient temperature to compensate for MCU internal osc. variation with temperature..pretty easy with a thermistor and one resistor.

author: nctnico
author_rank: Super Contributor
timestamp: May 17, 2015, 12:32:21 am
content: Strain gauge based weight sensors are cheap on Ebay. There are single chip solutions for amplification and A/D conversion. Add an MSP430 (from TI) and you can create something ultra low power (if you power off the AD converter and strain gauge).

author: remixed123
author_rank: Regular Contributor
timestamp: May 17, 2015, 01:59:13 am
content: Thanks for your responses, so looks like power consumption can be managed, these strain gauges do not take much, just send the power when measuring. Got it.I have seen these strain gauges, but they all look like they need to be built into a package to use. Similar to bathroom or kitchen scales......to avoid the difficulties of having to build this my self, I was hoping to find something that could be used without having to create a cantilever setup....just basically place on a platform and get results......this will be used with jars, around 4 - 6 inch in diameter

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: May 17, 2015, 04:54:47 am
content: You can interface Load Cell straight to "microconverter" like AD ADUc845 micro with 10 channel 24 bit ADC. That would be minimal in parts.I think you do need 24 bit ADC if you use load cell.

author: nctnico
author_rank: Super Contributor
timestamp: May 17, 2015, 06:46:48 am
content: Quote from: remixed123 on May 17, 2015, 01:59:13 amThat is not difficult. Just use 2 parallel aluminium plates with a weight measuring element in between. Just make sure to use some spacers so the weight element can bend freely (although you probably won't see it bending).

author: remixed123
author_rank: Regular Contributor
timestamp: May 17, 2015, 10:32:29 am
content: Thanks Alexei.....it is an unusual sensor to work with, measuring weight. It looks like I need to track down some experts in the field. The microcontroller I am working with has 12bit ADCs.If anyone else has any experience or opinion, happy to hear more.

author: remixed123
author_rank: Regular Contributor
timestamp: May 17, 2015, 10:34:14 am
content: Quote from: nctnico on May 17, 2015, 06:46:48 amThanks for the suggestion, are you aware of any examples of this on the Internet, to show me in a diagram or photo exactly what you are recommending.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 17, 2015, 12:33:16 pm
content: Quote from: remixed123 on May 17, 2015, 10:34:14 amhttp://lmgtfy.com/?q=diy+micro+weigh+scale+project&l=1

author: remixed123
author_rank: Regular Contributor
timestamp: May 17, 2015, 12:42:31 pm
content: Quote from: Mechatrommer on May 17, 2015, 12:33:16 pmThanks! That was awesome and done with style!

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: May 18, 2015, 09:19:04 pm
content: There in ADuC845 datasheet (http://www.analog.com/media/en/technical-documentation/data-sheets/ADUC845_847_848.pdf) on page 93 there is a complete design for load cell.

author: remixed123
author_rank: Regular Contributor
timestamp: May 19, 2015, 02:18:50 am
content: Quote from: Alexei.Polkhanov on May 18, 2015, 09:19:04 pmThanks for showing me that circuit for a load cell. I am assuming much of that is available within a commercial load cell, and I will just need to add an amplifier and before reading the signal on my ADC pins. I am not wanting to create a load cell.....just use one to measure weight.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: May 20, 2015, 03:48:04 am
content: Quote from: remixed123 on May 19, 2015, 02:18:50 amIf you are talking about ADUc845 then you can see on very first diagram in datasheet - there is already a PGA inside.

author: remixed123
author_rank: Regular Contributor
timestamp: May 20, 2015, 04:04:37 am
content: I just had a look at the ADUC845, nice microcontroller.....but expensive.....the product I am researching this for will need to be affordable, it will be sold in the 100s to 10,000s...

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: May 20, 2015, 07:39:09 am
content: Well, they are not very cheap. You can use version of TI MSP430 with 2x24 sigma-delta and PGA ... I would try to keep it down to single chip without separate ADC, amplifiers, current sources, antialiasing filters etc.Well I am not sure I understood your idea about the sensor completely - you want sensor to be self-contained and return weight value by I2C or you want to connect high res I2C ADC to your micro controller and do all conversions, calibrations and filtering in your micro?

author: remixed123
author_rank: Regular Contributor
timestamp: May 20, 2015, 07:55:36 am
content: I'm hoping to get this working with the ADCs that come with the Texas Instruments CC2640...which is around $4 at 1000 and it offers the Bluetooth I need for the solution. I work exclusively on connected devices, usually connected to the cloud.All calculation and communication will happen on a single chip, there will be no I2C involved. Calibration, if required, will be done via software....I just hope any difference between ADCs is close to linear.The load cell I am going to try and get working is this one -http://www.seeedstudio.com/depot/Weight-Sensor-Load-Cell-03kg-p-2086.html(These can be purchased for around $2.50 each in bulk.So all up cost should be kept down...what I do not known is how accurate I can get the load cell to work.Any recommendation on an Amplifier I should be using for this? Obviously something that was not too costly.