Digital gain control of Opamp.

 Embedded-Systems, Opamp-Circuits  No Responses »
Jan 192005
 
Digital gain control of Opamp.

The gain of U1 can be controlled by a digital binary 1248 nibble at ABC. The gain at digital 000 is unity or 1 and the gain at various stages are set by 4051. There are eight different gains as the steps of gain resistor network is chosen by 4051. The on resistance of 4051 channel around 100E gets added to U1 pin 2 internal impedance. You can use separate resistor networks with trimpots for each channel if you require but keep the networks total burden on U1 pin 6 to around 10K, not less than than. You can use Read More …

 Posted by admin at 5:57 PM

Crystal Oscillator – Parallel Resonant

 Digital-Circuits, Embedded-Systems  No Responses »
Nov 062004
 
Crystal Oscillator - Parallel Resonant

74HCU04 is a chip that was made for this purpose, HCT may not work for such a circuit. C1 and C2 can go to upto 33pF and R2 can be increased to make R2 * C2 = t. Time constant much less than the period T of the crystal T = 1/F . This is to remove higher frequency components in the Oscillator. The circuit above is a parallel resonant oscillator circuit. The Crystal works by the piezoelectric principle, piezo means pressure. The electric field causes the impedance of the crystal to change. The LP Record Player needle is the Read More …

 Posted by admin at 5:15 AM  Tagged with:

Battery Backup Supply

 Embedded-Systems, Power-Supplies  1 Response »
Oct 282004
 
Battery Backup Supply

This is a 9V power supply which will work even on power failure. It uses a rechargeable battery and regulators. A transformer with 15-0-15 AC volts output is required. In the first regulator U1 the output is lifted up by 1.4V and in the second regulator U2 by a resistor divider. In the second regulator the voltage across resistor R3 is 5V, so the current is 5V / 1K = 5mA this adds to the quiescent current of 5mA from the regulators ground terminal and flows into the resistors R1 and R2 in parallel which form 404 ohms, 10mA thru Read More …

 Posted by admin at 7:37 PM  Tagged with:

555 watchdog for uC and uP systems

 555-Circuits, Embedded-Systems  No Responses »
Oct 232004
 
555 watchdog for uC and uP systems

When the microcontroller hangs due to a spike, EMI or RFI etc. the 7555 will reset the uC, if proper power supply design is done above circuit can also give a clean power on reset, the above circuit you should modify to suit your design. Better still use a uC with watchdog built in like some atmel chips, or use the watchdog chips from maxim which can also do RAM battery management. The circuit was developed over a old TI application note, 7555 i think fairchild may be making it, 7555 is CMOS version of 555 timer, advantage of 7555 Read More …

 Posted by admin at 12:28 PM
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