{"id":6795,"date":"2020-05-29T13:08:10","date_gmt":"2020-05-29T11:08:10","guid":{"rendered":"https:\/\/www.nerdiy.de\/?p=6795"},"modified":"2022-01-20T13:01:09","modified_gmt":"2022-01-20T12:01:09","slug":"howto-espeasy-wemos-d1-mini-adc-adapt-to-another-maximum-voltage","status":"publish","type":"post","link":"https:\/\/nerdiy.de\/en\/howto-espeasy-wemos-d1-mini-adc-adapt-to-another-maximum-voltage\/","title":{"rendered":"HowTo: ESPEasy \/ Wemos D1 Mini - Adapt ADC to a different maximum voltage"},"content":{"rendered":"

You can measure voltages using the ADC on the ESP8266. This is practical, for example, to record a battery voltage and thus the charge level of the battery or to record brightness using an LDR and a series resistor.<\/p>\n\n\n\n

However, the ADC of the ESP8266 is somewhat special. Unfortunately, without additional external wiring, this can only determine voltages up to a maximum of 1V. It is therefore not possible to directly determine the voltage of a LiPo battery because the voltage range of a healthy LipO is from 3.2 to 4.2V.<\/p>\n\n\n\n

In order for the ADC to also be able to measure higher voltages, a trick is used. A voltage divider is used to scale the voltage to be measured down to the possible voltage range of the ADC.<\/p>\n\n\n\n

The ADC then measures voltages in the range from 0V to 1V, which represent a higher value on the actual voltage divider. For example, a voltage divider is installed on the board of the Wemos D1 Mini to measure voltages up to 3.3V. That's practical, but unfortunately it's not enough to be able to measure the voltage of a LiPo, for example.<\/p>\n\n\n\n

How you can extend this maximum voltage range of the ADC is described in the following article.<\/p>\n\n\n\n

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Requirements<\/h2>\n\n\n\n

Helpful articles:
<\/strong>Before you start with this article, you should have dealt with the basics of soldering. You can find information about this in the following article.
Electronics \u2013 My friend the soldering iron<\/a><\/em><\/strong><\/p>\n\n\n\n

Required tool:<\/strong><\/strong><\/p>\n\n\n\n

In the following list you will find all the tools you need to implement this article.<\/strong><\/strong><\/p>\n\n\n

\n \n \n \n \n \n \n \n \n <\/colgroup>\n <\/table>\n \n \n \n<\/div>\n\n\n\n\n

Required material:<\/strong><\/strong><\/p>\n\n\n\n

In the following list you will find all the parts you need to implement this article.<\/strong><\/strong><\/p>\n\n\n

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\n \n \n \n \n \n \n \n <\/colgroup>\n <\/table>\n \n \n \n<\/div>\n\n\n\n\n
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Safety instructions <\/span><\/h2>\r\n

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How does a voltage divider work?<\/h2>\n\n\n\n

A simple voltage divider actually always consists of at least two resistors. In the circuit diagram of the Wemos D1 Mini below you can see a voltage divider in the area marked red.<\/p>\n\n\n\n

If you are not interested in the function of a voltage divider, you can simply skip this paragraph. \ud83d\ude42<\/p>\n\n\n\n

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Source: https:\/\/www.reddit.com\/r\/esp8266\/comments\/9idpbe\/does_anyone_know_where_to_buy_genuine_wemos_d1\/<\/a><\/figcaption><\/figure>\n\n\n\n

Two resistors are connected in series there. The upper connection \"A0\" comes from the connection pin \"A0\" on the Wemos D1 Mini circuit board. The voltage to be measured (e.g. your LiPo) is therefore connected to \"A0\".<\/p>\n\n\n\n

The \"ADC\" connection then leads to the actual ADC of the ESP8266. <\/p>\n\n\n\n

\"\"<\/figure>
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Why are you doing all this?<\/strong><\/p>\n\n\n\n

A property of a series connection of two resistors is used here. <\/p>\n\n\n\n

For a series connection of two resistors, the following applies, among other things, to the ratio of the total voltage to an individual voltage at one of the resistors is equal to the ratio of the total resistance to one of the individual resistors. This sounds complicated at first but is explained below. \ud83d\ude42<\/p>\n<\/div><\/div>\n\n\n\n

First of all: As a formula the whole thing would look like this:<\/p>\n\n\n

Formula 1:          \"frac{R1+R2}{R2}=frac{U1+U2}{U2}\" <\/p>\n

or<\/p>\n

Formula 1.2:    \"frac{R1+R2}{R1}=frac{U1+U2}{U1}\"<\/p>\n\n\n

\"U1\" and \"U2\" are the voltages that drop across the resistor \"R1\" and \"R2\". \"U\" is the same as U1 + U2 because in a series connection of (simple ohmic) resistors, the individual voltages add up to the total voltage.<\/p>\n\n\n

In short, the following applies: \"U=U1+U2\"<\/p>\n\n\n

The same applies to the resistances, because the total resistance of a series connection also consists of the sum of the individual resistances. <\/p>\n\n\n

\"Rsum=R1+R2\"<\/p>\n\n\n

These formulas can now be used quite well in Formula 1 and you get the following:<\/p>\n\n\n

\"frac{Rsum}{R2}=frac{U}{U2}\"<\/p>\n\n\n

\"U1\" and \"U2\" are the voltages that drop across the resistors \"R1\" and \"R2\" respectively. \"U\" is the same as U1+U2 because in a series connection of (simple ohmic) resistors, the individual voltages are added together to arrive at the total voltage.<\/p>\n\n\n\n

To do this, the formula must be rearranged so that the total voltage \"U\" can be calculated from the given values. The rearranged formula results in<\/p>\n\n\n

Formula 2:\u00a0 \"frac{Rsum}{R2}*U2=U\"<\/p>\n

If you now insert the corresponding values here, the following results:<\/p>\n

\"frac{320k}{100k}*1V=3.2V\"<\/p>\n\n\n

With the resistance values used in the voltage divider, voltages of up to 3.2V can be measured on the ADC.<\/p>\n\n\n\n

The practical thing: If you change these resistance values, you can also measure higher voltages with the ADC. For example, below is the voltage of a LiPo.<\/p>\n\n\n\n

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Voltage divider on the Wemos D1 Mini<\/h2>\n\n\n\n

So now you know roughly how a voltage divider works and what it can be used for, among other things. With this knowledge, we now want to find resistance values for the voltage divider in order to be able to measure the voltage of a LiPo battery. Its final charging voltage is a maximum of 4.2V. So that we can reliably measure this upper voltage, it is advisable to choose a slightly higher voltage. Let's just say 4.3V.<\/p>\n\n\n\n

Examples of resistors R1 are given in the next paragraph for the usual voltage values of 4.3V, 5V, 9V and 12V. This way you can save yourself the math. \ud83d\ude42<\/p>\n\n\n\n

\"\"<\/figure>
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The goal is now to determine the resistance values for R1 and R2.<\/p>\n\n\n\n

At the same time, we already know that U=4.3V<\/strong> and U2=1V<\/strong> have to be. This means that to calculate a value using Formula 2, we only have one value left that we have to determine in order to be able to calculate the last open value. But let's make it easy here and simply set this value. \ud83d\ude42<\/p>\n\n\n\n

We first set R2 to 100kOhm and see what value we get for R1.<\/p>\n<\/div><\/div>\n\n\n

The change now results in Formula 1 including the collected knowledge<\/p>\n

Formula 3: \"frac{{color{Red}<\/p>\n

In this formula we know all (green) values except R1 (red). So we try to change the formula to R1 and get the following:<\/p>\n

\"1+frac{R1}{R2}=frac{U}{U2}\"<\/p>\n

simplified and rearranged, this results in:<\/p>\n

\"(frac{U}{U2}-1)*R2=R1\"<\/p>\n

and the same colored accordingly:<\/p>\n

\"(frac{{color{Green}<\/p>\n

In this formula we now know all the green values and the (red) value we are looking for is neatly on the right side of the equal sign. \ud83d\ude42<\/p>\n

The desired value for R1 can now be calculated by inserting:<\/p>\n

\"(frac{4.3V}{1V}-1)*100kOhm=330kOhm\"<\/p>\n

This results in a value of 330kOhm for R1. So if we replace the 220kOhm resistor installed on the board of the Wemos D1 Mini with a 330kOhm resistor, we expand the maximum measurable voltage range from 3.3V to 4.3V and can therefore also measure the entire voltage range of a LiPo battery. \ud83d\ude42<\/p>\n\n\n

\"\"<\/a>
This is the bottom view of the Wemos D1 Mini. Among other things, the voltage divider for the ADC is also installed there.<\/figcaption><\/figure>\n\n\n\n

Formula for calculation:<\/p>\n\n\n

\"(frac{U}{U2}-1)*R2=R1\"<\/p>\n