Ben, Do you know, or know of, a man at Duke University named Peter Malin? He is associated with the SAFOD project Which deals with seismic activity on the San Andreas Fault in California near where James Dean was killed. Jim -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of Benjamin Grant Sent: Saturday, February 14, 2009 6:53 PM To: Microcontroller discussion list - Public. Subject: [EE] Pulsemeter schematic Hi all, I'm trying to design a pulsemeter for the developing world. The requirements are that it is low cost(<$8 is the target but I have already conceded that is not going to happen), relatively robust and reusable. The probe I'm using is composed of velcro and some feltlike material. It doesn't keep out ambient light. I'm essentially using pulseoximetry technology but only the IR led, not the red LED, as i'm uninterested in 02 concentration. Anyway, I'm pulsing the IR led at 200 hz, measuring the signal with the LED on and LED off, and subtracting out the ambient light. I'm a biomedical engineering student but analog design is certainly not my strength. This is unpaid work for an NGO, so you're not helping me get an A or anything, so any help would be much appreciated. The problems that have been brought to my attention by more qualified engineers are I posted the schematic here 1.) the nominal output voltage of all my opamps is 0, and a single supply op amp can't drive the voltage all the way down to it's negative rail. To fix this issue, it was recommended i power all the op amps with -.3V on their negative rail. I tried this and my signal just goes down to -.3 V, so the same problem is met. Any suggestions on addressing this issue? 2.) Even if the actual output is 300 mV away from the negative rail, because of the bias current and the voltage offset of the opamps, the last amplification stage with gain of 3300 is likely to create a situation in which the voltage is again driven to the nonoperating region of the opamp. I actually don't know much about calculating the theoretical effect of maximum bias current, voltage offset, etc. and any reference on the subject would be great. 3.) I'm using an LED display(3, 7 segment displays), which likely will use more power than I'd like, as this device needs to be run on a battery. However, LCD screens were not feasible if using a low cost pic, and needing a low cost screen. I am strobing the LED digits, so I'm only powering one at a time. This part has been completed and appears to work, but any power saving advice would be appreciated. As this device needs to run off a battery, utilizing single supply solutions is preferable for sure. The output of this circuit will go into the pic16f684, assuming i can keep the program size small enough to fit on this pic. Thanks in advance for any suggestions regarding modifying this schematic to work properly. Also, the first part of the circuit was taken from an appnote from TI. However, the resistor values I chose, so the resistor values might be poor but the transimpedance amplifier circuit(consisting of the first two opp amps) should work in theory. The filtering and gaining was done by me, so they're an area of larger concern. Ben -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist