Electronic – thermistor in HC-SR501 PIR

References

Background: http://www.digikey.com/us/en/techzone/sensors/resources/articles/design-considerations-for-passive-infrared-sensors.html

More theory: http://media.digikey.com/pdf/Data%20Sheets/Cypress%20PDFs/CY3236A-PIRMOTION%20PIR,EVK.pdf

It looks like this is the sort of sensor: http://pdf1.alldatasheet.com/datasheet-pdf/view/191226/MURATA/IRA-E700.html

Answers

1. Changing the oscillator frequency will affect all the timings based on it, making them longer or shorter. For example going from 62.5Hz to 50Hz will make all the times about 20% longer. I would avoid changing it very far; it will give you worse results and the datasheet doesn't say what the max and min are.

2. Supplying your own voltage to the sensor will not change the references used by the comparator windows. My guess is that this would change the center of the temperature range. Fortunately, this isn't necessary: the IRA-E700 example sensor claims a voltage range from 2 to 15V, which is nice.

   The sensor isn't a digital device with a specific level, it's a lot more like an electret microphone: a sensing element connected directly to a MOSFET which amplifies its tiny signal.

   The reason why you get a positive and negative pulse is that there are two sense elements in the sensor, connected in opposite directions. One produces positive pulses when it "sees" human body warmth, the other produces negative pulses. So you have to move from one to the other to trigger it, but it won't trigger on changes in ambient temperature that affect both.

3. A push-pull output is an output that works in both directions, unlike e.g. open collector outputs which can only sink current. The \$V_{oh}\$ and \$V_{ol}\$ values are its output high and low voltage specifications.

4. R4: this sets the voltage gain in the pair of amplifiers. The signal from the sensor is tiny, and amplified quite a lot. Just how much is set by R4.

Generally it's much easier to buy a PIR module with integrated sensor and detector than try to combine the two yourself using this chip and some unspecified sensor.

The BISS0001 is a not well documented (In English) Chinese IC. How it operates with certain changes is not well known. But the changes you made shouldn't affect it's life.

In the posted schematic, R13, along with RT1 (The Time potentiometer) and CY1 control the On Time of the Module's output. R33 and CY2 control the Minimum Off Time. There is also Jumper JP1 which handles the Trigger mode.

The Formula for the On Time is Tx ~= 24576 * Rx in kΩ * Cx in µF. This provides time in milliseconds. Multiply by 0.001 for seconds. R13 + RT1 = Rx. RT1 can be anywhere between 0Ω and 1MΩ (1000kΩ). CY1 = Cx. In the Schematic, CY1 is a 103 capacitor, which is 0.01 µF. Given this, we can calculate the two time ranges, when you shorted R13 out (Value of 0):

Tx = 24576 * (0kΩ + 0kΩ) * 0.01 µF * 0.001 ~= 0 seconds
Tx = 24576 * (0kΩ + 1000kΩ) * 0.01 µF * 0.001 ~= 245 seconds

The Off Time is similar. Ti = 24 * Ri * Ci, again times 0.001 for seconds. Ri = R33, Ci = CY2 (104 means 0.1µF). Since you shorted R33 out...

Ti = 24 * 0kΩ * 0.1µF * 0.001 ~= 0 seconds

Multiplying by 0 is easy. That said, we don't know how the BISS0001 handles the time, if it has a minimum period or not. Other PIR sensor ICs use a RC circuit to create an oscillator, so shorting the resistor would break the oscillator function.

Also consider JP1, which connects pin 1, A (Mode Select). If tied to Low/ground, it will not retrigger if motion is detected while the output is already high/on. If tied to High/Vcc, it will retrigger, meaning the On Time clock restarts, even if the sensor is already outputting high. So if someone is moving around in the field of view, it won't turn off in the middle of movement.

Also, don't forget to turn the Sensitivity potentiometer to it's highest setting.

But assuming you have On Time set to 1 second or less, and Off Time set to 0 seconds, and sensitivity potentiometer set to MAX, you still have to deal with the PIR sensor. The BISS0001 handles detection logic, but the sensor itself is an analog device that has it's own conditions for signaling a change. Short of it is, depending on the fresnel lens you have on it, which helps widen and divide the field of view, which direction you have the sensor pointed at (people walking directly towards the front of it will be less noticeable compared to people walking across it's field of vision.), and how quickly the sensor detects the changes based on it's composition, it does take time for the module to trigger. The PIR sensor AND the BISS0001 logic chip both [likely] have hysteresis to prevent over sensitivity and chronic triggering. The sensor requires time to reset when first turned on, and map the field of visions' default Infrared State. It only maps changes from one "picture" to the next.