Page 20 IMSA Journal

Fire Alarm Simple CircuitsBy Jeff Alder, CET

Frequency to Voltage Converter

Introduction Hello and welcome to Simple Cir-cuits.

In this installment, I would like to review an IC that has been around since the early 1980’s. The IC is a frequency to voltage converter, but it is, in many ways different from many of the F to V converters I have worked with in the past.

The F to V converter part number is LM2907N-8. It is manufactured by Texas Instruments, and is available from Digi-Key for around $1.35.

It should be noted that the data sheet for this device refers to this part as a “tachometer circuit”. Indeed, the front end of the device has been specifically designed to interface with a variable reluctance magnetic pickup.

What makes this part so interesting however, is its flexibility. There are a variety of applications which, on the surface, do not have an apparent re-lationship with frequency to voltage conversion, yet employ the LM2907 to achieve their functionality.

The LM2907 comes in four con-figurations which I will address later in this column. The part is very straightforward to implement and is surprisingly versatile for a device that has been around for over 30 years.

Another useful aspect of this part is that it provides a logic output which activates once a pre-defined frequency has been reached.

To demonstrate the operation of this device, I bread boarded a simple “Speed Switch” circuit I found in the data sheet application notes. The circuit was wired as shown in

Figure 1. The LM2907N-8 is an 8 pin device and the number of exter-nal support components can almost be counted on one hand.

Figure 1 – Speed Switch Circuit Diagram

IC OperationThe LM2907N-8 has an internal structure as shown in Figure 2.

Figure 2 – Internal Block Diagram – LM2907N-8

The device contains a front end op amp/comparator circuit, with the negative side of the op amp termi-nated to ground within the IC. The LM2907N-8 therefore provides a single ended (ground referenced) tachometer input.

The front end amplifier drives a positive feedback flip-flop circuit, which in turn drives a charge pump circuit. The charge pump then converts the input frequency to a DC voltage, by imposing a current though an output (load) resistor (R1).

The operation of the charge pump is determined by the selection of a timing capacitor (C1) and the output resistor (R1).

The formula for calculating the out-put voltage Vout (the voltage across R1) is provided as follows: Vout = Vcc x fin x C1 x R1 x K

(K is the gain constant, which the data sheet defines at a typical value of 1.0)

We will come back to the impor-tance of this formula shortly.

The output voltage from R1 is then applied internally to a comparator circuit which, in turn, drives an on board “floating” output transistor. Because the output transistor is unreferenced, it can be connected to either sink or source an external load, with drive currents up to 50mA.

Speed Switch Circuit Component SelectionThe circuit design for the speed switch is very straight forward.

The data sheet provides a formula that is used to derive the necessary component values, based on the de-sired input frequency that will cause the logic output to change state. The formula provided is as follows: fIN ≥ (1/(2R1C1))

- fIN is the proposed frequency at which to switch the output transistor

- R1 is the output load resistor of the charge pump circuit

- C1 is the timing capacitor for the charge pump circuit

If we arbitrarily choose 1000Hz as the switching frequency and 220KΩ as the value for R1, we calculate an

Continued on page 22

Page IMSA Journal22Continued on page 24

The wireless world

we once only dreamed of …

It’s way better than we imagined !Delivering the future of wireless connectivity today.

3750 Arapaho Rd, Addison, Texas

www.encomwireless.com for more information.

Simple Circuits . . . Continued from page 20

approximate value of 0.0022uF for the timing capaci-tor C1.

These values calculate an actual switching frequency of 1033Hz, which should demonstrate the application nicely.

A Vout filter capacitor (Cf) is also required. This capacitor is placed across the output resistor R1. The size of this capacitor will determine the amount of output ripple across R1, and affect the response time for a new output voltage to stabilize.

For prototype purposes, I arbitrarily selected a 0.22uF monocap for the Vout filter capacitor Cf.

Speed Switch Circuit OperationThe LM2907N-8 requires zero crossings at its input to operate. It has also been designed to provide 0 Volts of output voltage when the incoming frequency is 0Hz.

I set my frequency generator to generate a ±1Vpp square wave, and applied the signal between input pin 1 of the LM2907, and pin 8, the ground reference.

R1, C1 and Cf are connected as shown.

The R2 and R3 values are defined in the application note as 5KΩ. These resistors form a resistor divider, establishing a switchover voltage reference at the input of the internal comparator of ½ Vcc.

In my circuit, Vcc = 10VDC. Therefore, ½ of 10VDC = 5VDC switchover reference voltage.

So let’s confirm the values of R1 and C1 using the origi-nal formula for calculating the output voltage across R1. V

out = Vcc x fin x C1 x R1 x K

Vout = 10V x 1000Hz x 0.0022uF x 220000Ω x 1

Vout = 4.84 VDC @ 1000Hz

Please note, that the circuit’s power supply voltage (Vcc) is a critical parameter within this calculation. An unstable power supply for this circuit could dramati-cally alter the output voltage generated by the front end, into the comparator circuit.

A highly stable power supply is a common require-ment with all the V to F and F to V devices I have used previously.

Because I used a LM7810 for my power supply regulator, circuit operation should remain relatively constant.

Obviously, R1 and C1 component tolerances will also influence the output voltage level, so some tweaking of R1 may be required to achieve an exact switch over frequency of 1000Hz.

I used a 680Ω resistor (R4) and high brightness LED as my load, in the open collector configuration of the output drive circuit.

The circuit worked quite well as soon as I powered it up.

The switching frequency was a little off, which was to be expected, but as soon as I tweaked the value of R1, the circuit worked perfectly and was very stable, even moving back and forth across the transition fre-quency.

I monitored the output voltage with a scope across R1, and could watch it rise in a linear manner as the input frequency was increased.

The 0.22uF allowed for 50mV of ripple voltage across R1. The ripple waveform was 2 x the input frequency, and presented as a sawtooth waveform (as expected). The 50mV ripple voltage amplitude remained constant over the entire range of the input frequency.

(Please See Figure 3)

Figure 3 – Input Frequency vs. Charge Pump Ripple Output

Additional ApplicationsAs mentioned earlier, this device is particularly versa-tile. The applications notes provided in the data sheet include circuits for the following applications:

- Tachometer- Speed Switch (reviewed in this column)- Breaker Point Dwell Meter- Engine RPM Meter- Capacitance Meter- Touch Switch Circuit- Overspeed Indicator- Automotive Anti-Skid Circuit

As I also mentioned earlier, the LM2907 comes in four configurations. The part numbers for these configura-

Page IMSA Journal24

tions are as follows:- LM2907- LM2907-8 (reviewed in this column)- LM2917- LM2917-8

The LM2907-8 is an 8 pin device which provides inter-nal connections that commit the frequency input to a ground referenced mode of operation, and commit the output voltage from the charge pump to the positive input of the internal op amp/comparator. This part does not include an internal shunt regulator which would regulate an unstable power supply voltage.

The LM2917-8 is an 8 pin device which provides in-ternal connections that commit the frequency input to a ground referenced mode of operation, and com-mit the output voltage from the charge pump to the positive input of the internal op amp/comparator. The LM2917-8 does include an internal shunt regulator that will regulate an unstable power supply voltage.

The LM2907 is a 14 pin device which does not make the aforementioned input signal ground commitment. This part is a truly differential input device which also does not commit its internal op amp/comparator to the output of the charge pump. This allows the designer to implement the op amp as an active filter in support of the application.

This differential input device does not however, pro-vide on board input protection. It is therefore impera-tive that input signals are properly terminated and do not exceed specified levels. More information pertain-ing to the design around these parts is available from the data sheet.

The LM2907 does not include an internal shunt regula-tor which would regulate an unstable power supply voltage.

The LM2917 is a 14 pin device which does not make the aforementioned input signal ground commitment. This part is a truly differential input device which also does not commit its internal op amp/comparator to the output of the charge pump. This allows the designer to implement the op amp as an active filter in support of the application.

This differential input device does not however, pro-vide on board input protection. It is therefore impera-tive that input signals are properly terminated and do not exceed specified levels. More information pertain-ing to the design around these parts is available from the data sheet.

The LM2917 does include an internal shunt regulator that will regulate an unstable power supply voltage.

Simple Circuits . . . Continued from page 22

ConclusionDon’t be afraid to give this part a try. It bread boards quickly and performs as described.

I have only scratched the surface of this part and its potential applications.

As always, be sure to read the datasheet thoroughly to ensure that any applications you may have in mind, are suited to the device.

Other stuff…I have been recently experimenting with another device and producing some interesting patterns on my oscil-loscope. It is something I have been doing for purely entertainment purposes, as it is not an appropriate application of the device.

That said, I am including a couple of patterns that I have generated for interests sake. Please see Figures 4, 5 and 6.

Figure 4 - Lissajous Pattern # 1

Figure 5 -Lissajous Pattern # 2

Figure 6 - Lissajous Pattern # 3

If there are any readers who would be interested in a column describing how I have been able to generate these patterns, please drop me a line at j.alder@shaw.ca. If I get enough interest, I will try to put something together for an upcoming issue.

Until next time, Take care out there!

TRAFFIC & PARKING CONTROL COMPANY, INC.

Superior LED Light QualityExceptional Energy SavingsUnmatched 10-Year Warranty

Check out TAPCO's On-Line Store and Catalogs of Products!

Low Maintenance, Long LifeOutdoor White Cree XTE LEDDark Sky Compliant Design20 Year L70 Output RatingRetrofit Payback in 3 Years!

13070004.indd 1 7/1/13 2:16 PM