|
7555 type oscillator used for frequency output from a capacitive sensor and radiometric duty from thermistor.
Shunt reference used because they are unconditionally stable (regulators need capacitors and that was causing problems).
The 555 discharge pulls 6.9mA from a current source. The shunt reference is supplied from a 2.2mA current source.
Transmission line elements are in four parts, that total 100 meters of a CAT5 pair.
The 180pF is a capacitive sensor, e.g. humidity http://www.meas-spec.com/downloads/HS1101LF.pdf
The 470k is a thermistor, e.g.
http://www.murata.com/products/catalog/pdf/r44e.pdf
The 7555 is like a NE555 but much better, for example it should work at 5V where 555 will not. CMOS logic gates help the output look more like the 7555. http://www.intersil.com/content/dam/Intersil/documents/fn28/fn2867.pdf
Current flow over the CAT5 pair alternates between 3mA and 10mA. The 100 Ohm is termination. Near the MCU an NPN turns the termination signal into a voltage pulse that an MCU can read.
I've seen the 555 model have issues when it isn't directly ground.
The 180pF capacitive sensor could be a humidity, level, or pressure sensor. For level sensing just use another CAT5 pair which has 50pF per meter in air, and 88 times that in water (e.g. 30 cm in water runs at about 1kHz). Capacitive pressure sensing is another option.
http://epccs.org/indexes/Board/HT/
UPDATE: What if I have a 20mA current source... I don't see a difference.
My control board has a 20mA source for loop power.
http://epccs.org/indexes/Board/RPUno/
|
