# Reed Switch Whitepaper
### Introduction
The reed switch is an **electrical switch operated by applying a magnetic field**. Without the magnetic field, the switch is open, with the magnetic field, the switch closes. This sensor allows Simple Hardware devices to cover a great number of use cases - **when combined with a magnet**.
{% hint style="info" %}
The reed switch replaces the button functionality in the **SimpleIndustry and SimpleMeter** devices and is used to trigger **downlink**!
{% endhint %}
**There are three API 6 user modes that work with the sensor:**
* Reed switch on/off
* Reed switch counter
* Reed switch pipeline
Each of these modes covers a different **business logic** by having sending **different data** based on **different triggers**. Let's have a closer look.
{% hint style="success" %}
**IMPORTANT: The reed switch functionality below works exactly the same with switch/dry contacts!**
{% endhint %}
### Reed switch on/off
**How it works**: When the reed switch **closes**, a message is sent **immediately**. When the magnetic field is no longer present and the switch **opens**, a different **instant message is sent**. Very simple!
{% hint style="info" %}
Reed switch on/off is also available in three different WiFi versions. You can read about them [right here!](/kb/pre-sales-support/untitled/location-tracking-whitepaper.md)
{% endhint %}
### Reed switch counter
**How it works**: The device will count the number of **times the reed switch has closed** (number of magnetic field presences/magnet rotations). You can choose the **trigger** of the message: either it's **time** (number of switches in e.g. one hour) or the exact **count** (message sent after e.g. 50 switches.)
### Reed switch pipeline
**How it works**: The device will measure how long the reed switch **was open** and then how long it **was closed**. This will then be reported in a message that contains a total of **8 time periods**: 4 for both states. E.g. reed switch closed for 12 minutes, then opened for 10 minutes, then closed again for 8 minutes, etc.
### Reed switch modes comparison table
| Mode | Reed switch on/off | Reed switch counter | Reed switch pipeline |
| ------------------------- | ------------------------- | ---------------------------- | --------------------- |
| Mode hex (API 6) | 0x0C | 0x13 | 0x08 |
| WiFi versions available | ✔️ | ❌ | ❌ |
| Message trigger | switch open/closed | time/count reached | time - pipeline full |
| Message availability | instant | customizable as per settings | when pipeline is full |
| Appended payload possible | ✔️ (not in WiFi versions) | ❌ | ❌ |
### API 6 downlink register related to the reed switch
Reed switch-based modes can be fine-tuned with these downlink registers:
| Register name | Register hex | Register value range | What it is and what it does | Useful for |
| ---------------------------------------------------------- | ------------ | -------------------- | ---------------------------------------------------------------------------------------------------- | --------------------------------------- |
| Sleep time | 0x04 | SimpleTime | Time after record/message during which the device will ignore any sensor input. | Reed switch on/off, Reed switch counter |
| Reed switch measurement period interval | 0x37 | SimpleTime | How often the processor wakes the sensor up and asks for data input - how often the sensor measures. | All reed switch modes |
| Reed switch pipeline time threshold for exponent selection | 0x46 | SimpleTime | Please see the mode documentation for detailed information. | Reed switch pipeline |
| Reed switch detection delay | 0x4B | SimpleTime | How long should the change of state last before the message is triggered or data recorded. | All reed switch modes |
| Reed switch counter count trigger | 0x50 | Number 1-255 | After how many reed switches should the message be sent. | Reed switch counter |
| Reed switch counter time trigger | 0x51 | SimpleTime | After how long should the message be sent. | Reed switch counter |
[Here's the API 6 table for more information!](https://docs.google.com/spreadsheets/d/1hGcSu_Wcld7ZAccNz29Ni4D3gWtkkKHsiGtZQLGJE6g/)
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