Automatic cleaning of button contacts in a microcontroller device. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer
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As you know, buttons and other devices with electrical contacts have the effect of their "bounce" - uncontrolled multiple closing and opening in the first 20 ... 40 ms after the application or removal of mechanical force. From the point of view of physics, this is a series of local breakdowns of a thin oxide film covering the contact surfaces. The situation is aggravated if the contacts operate in difficult climatic conditions, for example, in a warm, humid atmosphere, as well as in the presence of dust and pollution.
To reduce the resistance of closed contacts operating at low values of the switched current, it is useful to periodically pass current pulses of increased amplitude through them, to some extent cleaning the surface from oxides and wear products. For example, the MAX13036 chip used to collect information about the status of various contact sensors installed in a car works according to this principle. In its technical description [1] there is a special parameter "Wetting current", which means "wetting current" in English.
Fig. 1
In devices based on microcontrollers, it is also possible to organize the removal of an oxide film by a current pulse, moreover, in the process of determining the state of the contact. On fig. Figure 1 shows a typical diagram of connecting the SB1 button to the IAC line of the microcontroller port with a load resistor R1 and a low-resistance resistor R2, which limits the amplitude of the "wetting current" pulse generated by the microcontroller. 2.
Fig. 2
Initially, the IAC line is in input mode. The program periodically checks its status as soon as a low voltage level is detected here, makes an anti-bounce pause of 50 ms. If the level remains low (that is, the SB1 button is really pressed), the program switches the PXX line to the output mode and sets it to it is high level, forming a "wetting current" pulse. Then the program returns the IAC line to the input mode and works further according to the algorithm embedded in it. Of course, the positive cleaning effect will appear only when the button is pressed next.
The resistance of the resistor R2 is selected experimentally depending on the design and material of the contacts. For example, for a MAX13036 microcircuit, the amplitude of the "wetting current" pulse can be in the range of 7,5 ... 40 mA, its duration is 10 ... 35 ms, the duration of the drops is 1 ... 6 μs. Therefore, when the microcontroller supply voltage Upit = 5 V, the resistance of the resistor R2 should be 100 ... 620 Ohm.
This resistor is also useful at the prototyping stage, even without passing a current pulse through it. It will save the microcontroller from damage if a bug in the program being debugged puts a port line that should be an input into output mode.
Fig. 3
On fig. 3 shows another common button connection diagram. Here, to save elements, a load resistor Rp is used, located inside the microcontroller. The operating current through closed contacts does not exceed tens or hundreds of microamperes.
The operation of contacts on microcurrents has its own characteristics [2]. In particular, arbitrary changes in the resistance of closed contacts with time can be observed, which is explained by the growth of sulfide and polymeric oxide films on deformed microprotrusions of the contacting surfaces. Installing a resistor R1 and passing the "wetting current" in such a case is highly recommended. The exception is buttons whose contacts are made of precious metals or coated with them. They are not susceptible to oxidation and usually work well at low current.
Fig. 4
The table shows a program fragment in the Wiring language for a microcontroller module of the "Arduino" family. A self-made module belonging to this family was described in [3]. To the D2 line of the module according to the circuit shown in fig. 4, the SB1 button is connected. In this case, it is not connected to a common wire, but to the power plus. This is done to show the absence of fundamental differences in the current pulse generation algorithm. Time relations correspond to fig. 2, only the low and high levels are reversed.
Literature
- MAX13036 Automotive Contact Monitor and Level Shifter. - datasheets.maxim - ic.com/en/ds/M AX13036.pdf .
- Levin A. Contacts of electrical connectors of radio-electronic equipment. - M.: Soviet radio, 1972.
- Ryumik S. Microcontroller module "InterDuino". - Radio, 2010, No. 10, p. 27-30.
Author: S. Ryumik
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