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Michael Karas
03/26/12 08:00
Read: 587 times
Beaverton Or

#186889 - Lack of resistors.
Responding to: ???'s previous message
You should re-review your design. I took the opportunity to look over your schematic and note that you seem to have a dislike for resistors. Here is a list if things I think would be essential to successful deployment of your circuit.

1) On your keypad there should be pullup resistors on the side of the switch matrix that will be used as inputs. That could be either the columns or rows depending upon how you write your scanning code.

2) Your interrupt pin should have a pullup resistor on it as it comes in from a connector. Even more essential to have a pullup if the smoke detector has an "open collector" type output.

3) There should be a pullup resistor on the base of the Q1-LED transistor to ensure sufficient base drive to saturate the NPN when conducting the LED current.

4) There needs to be a series current limiting resistor with the yellow LED. (You apparently already knew of this need because you have a similar series resistor on the green LED in the power supply circuit.

5) There will need to be a pullup resistor in the base of the Q2-SPEAKER transistor to ensure sufficient base drive to saturate the NPN when conducting the SPEAKER current.

6) Evaluate the speaker carefully. What type of series DCR does it have? You may have to make sure that Q2 has a rating up to the job. Or else put some resistance in series with the speaker to reduce current to a level safe for the transistor.

7) The OUTERSW connection should have a pullup resistor.

8) The INNERSW connection should have a pullup resistor.

9) The REEDOUT connection should have a pullup resistor.

10) The REEDIN connection should have a pullup resistor.

11) Many LCD modules require some series resistance between the A and K backlight connections to limit current to a safe level.

12) It is recommended to have some pullup resistors on the inputs to the ULN2003A part to ensure good saturation of the driver transistors inside the part.

13) Some board assembly and test manufacturers will insist on some additional resistors to support in-circuit testing. This would include a pullup resistor on the EA pin.

14) There should be a series resistor with the RESET switch so that repeated discharges of the C3 capacitor do not eventually destroy the switch contacts.

15) Not related to resistors but your design is devoid of showing bypass capacitors. It is essential to bypass across the VCC and GND connections of the MCU (which you do not even show on the schematic). Additional bypass on VCC would be recommended at the high side of the SPEAKER.

16) And finally, while not related to the same subject you should consider trashing that R/C reset circuit and replacing it with a proper reset supervisor device.

Michael Karas

List of 12 messages in thread
Interrupts, the 8052 & Solenoids      Cian Young      03/24/12 13:33      
   Freewheeel diodes      Per Westermark      03/24/12 15:00      
      ULN2003a      Cian Young      03/24/12 18:34      
         more than one inductive load here      Jim Granville      03/24/12 19:11      
            Solenoids      Cian Young      03/24/12 19:26      
               diode choices      Jim Granville      03/24/12 19:42      
         Diodes in ULN2003a      Michael Karas      03/25/12 08:21      
         Within this chip      Erik Malund      03/26/12 07:03      
            Do I hear...      Michael Karas      03/26/12 07:07      
   Irish keypad layout!!      Andy Neil      03/25/12 06:27      
   Lack of resistors.      Michael Karas      03/26/12 08:00      
      Success      Cian Young      03/28/12 11:00      

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