Ask the Engineer
Frequently Asked Application Schematics
Ask The Engineer is a collection of frequently requested application schematics for electronics designers and experimenters needing examples of typical interfacing circuits for magnetic sensors. The depictions are done in an "engineer's notebook" fashion for brevity.
Battery Powered Current Sensor
- To limit the earth's magnetic field influence on the current measurement, consider placing the sensor and conductor in a magnetically shielded enclosure.
- Calibrate the measurement circuit by forcing a known conductor current and measuring the resulting voltage output. If sensor to conductor displacement changes, recalibrate
- Use the trim pot to null the sensor offset, for a zero volt output with no conductor current.
- This sensor circuit can measure DC and AC currents to the bandwidth of the op-amp. Additional bandwidth limitation can be had by placing capacitance across the 475k-ohm feedback resistor.
- Other power supplies can be used with this circuit if the selected op-amp can operate. Note that the 1N4148 diode can be removed if reverse polarity protection is un-needed.
- Magnetic fields larger than 20 gauss at the sensor may require a set/reset circuit to restore performance.
- Consult with application note AN-209 to further understand the principles of current sensing via magnetic fields emitted from conductors.
- Simple circuit for laptop computers, portable devices, etc. for detecting change in location (i.e. rotational movement). The HMC1021S is placed on a horizontal circuit board in a magnetically clean area.
- 10-bit to higher ADC’s recommended for best resolution. Software should allow for slow drifts in value, but trigger for disturbances if sudden value changes in many ADC counts.
- Gain set at 200 like compass circuits, but can be from 100 to 300 V/V or wider if desired.
- Both digital and manual set pulse switches shown. NPN transistor must be high beta.
- This design can be used at 3.0 or 5.0 volt power supplies
Simple Magnetic Gradiometer
- Gradiometers are basic magnetic anomaly detectors, commonly used for metal detection. The near sensor is the reference in which the far sensor is used to probe for metals. Differences show up in the meter.
- Position of the HMC1021S sensors must be well aligned in the rod axis.
- Voltage Supply (V+) can be from 3 to 12 volts, depending on quad op-amp supply range.
- Rmeter selected based on chosen meter movement. Typical values are from 500-ohms to 10k-ohms.
- Large temperature changes may require re-adjusting the offset trim pots.
- Magnetic fields larger than 20 gauss may require a set/reset circuit for each HMC1021S to restore performance.
- Multiple axis gradiometers may be built by replicating this circuit and using the HMC1022 or HMC1023 sensor arrays.
2-Axis Wireless Phone Compass Circuit
- This circuit is optimized for voltage operation and using the HMC1052 AMR sensor in the 1.8 to 3.6 volt battery supply. To conserve battery life, you can power off the sensor and amplifier supplies (VDDSW) when not measuring the magnetic axes. The set/reset MOSFETs can remain powered and will not draw current unless the gate logic toggles.
- The op-amp and MOSFET types are not specified to allow sourcing from multiple vendors. MOSFETs should have Rds(on) ratings of 0.4 ohms or lower per FET. All parts should be chosen for low voltage compatibility.
- Capacitors C3 through C5 should be chosen for low ESR performance. Stacked ceramic film types preferred for 0.5 ohm ESR nominal.
- Placement of the HMC1052 must permit a standoff from other SMT parts and ferrous metals for best performance. Compass accuracy is set primarily by processor ADC resolution and firmware to compute heading. Typically, +/- 3 degree accuracy or better can be obtained.
Set/Reset Strap Driver – HMC2003
- Two IRF dual-FET used in a "H" bridge set/reset strap driver.
- Logic input is normally high with a high-to-low transition for a reset pulse, and a low-to-high transition for a set pulse.
- Capacitors should be Low-ESR type (<0.5 ohms) for low voltage loss.
- The regulator circuit is the designer's choice for +10.0 volt operation deeding the 100-ohm charge resistor.
- Other FET switches can be substituted with care for logic interface, low rds and supply voltage.
- Inverters should be chosen for 10-volt supply operation.
- TTL to CMOS level shifter may be composed of either a logic transistor, comparator, or dedicated logic shifter.
6-Axis Compass with SiRFstar3 Chipset
- Voltage range is +2.8 to +3.15 volts, confined by GPS chipset.
- Substitution of 3-Axis Accel, ADC, and magnetic sensors can be permitted for brand preferences, cost concerns, and layout. Host controller may have onboard ADC channels to further minimize chip count and costs.
- Keep the layout tight to minimize extra supply decoupling capacitors. A 10uf reservoir capacitor is recommended if supply (battery, regulator) is more than 2 inches away