DE4119406A1 - Transmission photometer with holder for specimen cuvette - has LED illuminating cuvette, sensor photodiode and signal amplification, conversion and display system - Google Patents

Abstract

The specimen cuvette (K) is illuminated by a LED (PL1,PL2). The light is fed to a sensor photodiode (PS) after attenuation and/or modification by the contents of the cuvette. The photodiode signal is amplified and converted for display in analogue and/or logarithmic and/or digital form. The LED is supplied with a frequency higher than a background light frequenc via a current defining resistor (R1,R2). The photosensor signal is filtered by a high pass filter (BP) with d.c. suppression whose pass band contains the supply frequency. The filtered signal is fed to a dual-path rectifier (GL) followed by a low pass filter (TP) whose output is displayed. ADVANTAGE - Simple transmitted light photometer of low wt., low current consumption. Easy to use yet sufficiently accurate.

Description

The invention relates to a photometer with a receptacle for a sample cuvette with the light of one Light-emitting diode is applied, on the other hand, the Sample cell weakened by its content and / or changed a sensor photodiode is supplied, the Photosensor signal amplified and then analog and / or logarithmic and / or digitally converted one Display device is supplied.

Such transmitted light photometers are known. You assign one high ambient light sensitivity so that they each must be carefully shielded during the measuring processes if not with very high light intensities from expensive ones Light sources are operated, which is also an undesirable Heating or conversion of the sample can occur in the photometer can.

It is an object of the invention, a simple one Transmitted light photometer to create that light weight, low power consumption, convenient handling and sufficient Accuracy.

The solution is that the LED by a Feed frequency that is much higher than usual one Interference light frequency is above a current-determining Resistor is fed and the photosensor signal by means of a high pass with a level suppression is filtered, whose pass frequency range is Includes feed frequency, and then one Full-wave rectifier with a downstream low-pass filter is supplied, the measured value output signal for display brought.  

Advantageous configurations are in the subclaims specified.

The pulsed loading of the LEDs allows it in connection with the frequency selective amplification of the Photosensor signals with a very low light output to achieve high measuring accuracy regardless of a Ambient lighting is.

The pulse frequency of the light source is appropriately so much higher than that of AC or three-phase lighting occurring 50 to 300 Hz interference frequency selected that a It is easy to filter out the interference frequencies.

The pulse signal of the supply voltage is advantageous rectangular and advantageously has a pulse-pause ratio from 1: 1 to. The filter on the receiver side is also like this dimensioned as a bandpass that the rectangular shape of the received useful signals are essentially retained, annoying flashes of light are filtered out.

The rectangular shape of the signals and the selected one Pulse-pause ratio in connection with the subsequent rectification a faithful reproduction of the Useful signal amplitude of the receiver, since the rectangular portions rectified above and below the mean Result in the same signal, one in the preceding Filtering still low low frequency Extraneous light signal component in a low-pass filter practically completely is eliminated, that only the direct signal unhindered happens. The upper low-pass cutoff frequency is therefore below the stray light frequency of 50 Hz.  

The accuracy of the photometer can be increased by the standard light-emitting diodes narrow-band interference filter downstream. It is intended to be switchable one of several LEDs each different To feed light frequencies with the square wave voltage, whereby the current determining the light intensity through one associated series resistor is specified so that all LEDs when using a neutral blank generate the same useful signal at the receiver. The light emitting diodes are all closely adjacent to each other in the reception area of the Sensor diode arranged.

The simplicity of operation is achieved in that a Control device ensures that one after each Switching to another light emitting diode is an indication of Switchover takes place, and a new reference sample measurement is initiated. Furthermore, each time a sample is changed Cell contact signal sent to the control device, the controls that the first measurement after a Switchover to generate a reference value evaluated is done by an analog-to-digital conversion and a digital one The measured value is saved. This saved The measured value is converted as a reference value again and at the analog-digital conversion of the following measured values Transducer fed so that a relationship is formed immediately the reference value for the respective measured value during conversion he follows.

Furthermore, it is advantageously provided in the Digital conversion of the ratio value the successive Integration of the partial values using one To make exponential function element. This is what Integration capacitor a discharge resistor in parallel switched so that a during the conversion process  exponentially decaying discharge voltage arises. This is the digital result of the follow-up measurements each the logarithmic ratio of the stored Reference value and the respective measured value are displayed.

Furthermore, a linear-logarithmic converter for the Anlaoge measurement signal provided so that this immediately can be displayed and output. This is off A simple circuit limited due to effort Accuracy provided.

The device is so small and light that it can be used as a Suitcase device is designed with battery power and because the low power consumption means a long battery life he brings.

An advantageous embodiment is shown with reference to FIGS. 1 to 3.

Fig. 1 shows an overview block diagram;

Fig. 2 shows a block diagram of the lighting and sensor electronics with the measurement signal processing;

Fig. 3 shows a block diagram of the Meßwertwandlung and processing.

Figure 1 shows an overview circuit diagram of the photometer. In the rectangular pulse generator (RG) with a supply frequency (SF) of z. B. 440 Hz voltage pulses of a 1: 1 duty cycle, which are guided to a selector switch (WS). The respectively selected switch contact supplies a light-emitting diode (PL 1 , PL 2 ) via a current-determining resistor (R 1 , R 2 ). These light-emitting diodes are arranged on the one hand of a holder for a sample cuvette (K) and on the other hand a photosensor diode (PS) is arranged. Their signal is fed to an amplifier (V) and then to a bandpass filter (BP), which preferably passes the square-wave signals of the feed frequency and filters out the stray light frequencies and a DC level that are caused by extraneous light and amplifier drift. The filtered signal is rectified in a rectifier circuit (GL), which is designed so that it has a low drift, and subsequently filtered in a low-pass filter (TP), so that the square-wave signals with their duty cycle 1: 1 are a DC, seamless analog measurement signal result that the frequency is largely cleaned of residual extraneous light signals.

A control device (ST) are the signals of the Selector switch (WS) supplied, each of which at a Switching arising signals each a control signal (ST1) generated, with which a first measurement evaluation of a Reference sample through a symbol display on the Display device (LCD) is requested and the evaluation of the reference sample measurement signal is prepared. Farther is one of a button (T) on the cell holder Signal line led to the control device (ST), the due to the button signals that change when changing the cuvette Connection to an occurrence of the first control signal arise, each a special reference measurement evaluation controls with a second control signal (ST2).

In the reference measurement signal evaluation, a Digital conversion and digital storage of the Measurement signals made in a memory (SP).  

In the following measurement signal evaluations, the Memory content as a reference value (REF) Analog-digital converter (ADV) supplied, the input of which the filtered and rectified measurement signal is fed and its output switched to the digital display (LCD) is.

Furthermore, the analog, processed measurement signal is via a Logarithmizer (LOG) led to an output.

Interference filters (I 1 , I 2 ) are arranged in the beam path of the light-emitting diodes (PL 1 , PL 2 ) and promote the measuring accuracy.

Fig. 2 shows further details of the circuit arrangement. For the supply, a battery power supply is provided which is stabilized with a first Zener diode (Z 1 ) and which feeds all the circuits and the square-wave voltage generator (RG) which feeds the light-emitting diode selector switch (WS).

The photosensor (PS) feeds the amplifier (V), whose Signal the adjustable bandpass (BP) feeds at its output a DC disconnection is made. The AC measurement signal is in the low offset rectifier (GL) rectified and by a low-pass filter (TP) smoothed, whose cut-off frequency is below 50 Hz.

Fig. 3 shows details of the signal converter and the memory (SP). The latter consists of a binary counter (BZ) which, in conjunction with the digital-to-analog converter (DAU), records the reference measured value controlled by the second control signal (ST2). For further measurements, the stored reference value is then converted to analog at the reference input (REF) of the analog-digital converter (ADC), the output signals of which are made visible on the display (LCD).

The first control signal (ST1) controls on the Display device (LCD) an icon by which the Operator is alerted that the next one Measurement with a reference sample.

The integration capacitor (CI) of the converter (ADU) is bridged with a discharge resistor (RE), so that a logarithmic signal conversion takes place. The digital-to-analog converter (DAU) is fed by a post-stabilization circuit with a second Zener diode (Z 2 ). This is followed by a voltage regulator (P) with an impedance converter. The post-stabilized voltage serves as an accurate reference voltage for the digital conversion of the reference measured value. A temperature-stabilized version of the Zener diode is expediently used.

It is in the skill, the functions of the Filtering, rectification and signal processing in one Microprocessor, or with commercially available Realize filter modules. To do this, the analog Measuring signal directly behind the amplifier (V) via a Digital converter led and from the processor to Further processing added. By appropriate Offsetting the reference measurement value with the following ones The relationship to the reference measurement signal can be measured form from it. A subsequent conversion of the ratio in a logarithmic value does the same user-friendly display result like the one shown Contraption.  

The advantages of the device are again here compiled:

• - small size, thus portability,
• - no glass optics, therefore shock resistance,
• - low power consumption, thus by means of a single one 9 V battery possible power supply for more than 20 hours,
• - high precision of the measurement results,
• - Digital display of extinction, thus avoiding Reading errors,
• - Insensitivity to stray light, thus eliminated a cover flap over the cuvette shaft, thereby simple and quick handling,
• - automatic comparison to reference sample, thus Operator safety,
• - only two controls (selector switch for measuring wavelength and switch), thus ease of use.

The following dimensions are used in practical operation proven to be advantageous: the rectangular generator pulses the LED current with a 50% duty cycle at approximately 440 Hz. The currents of the light emitting diodes because of the Use of such types with high efficiency are only 0.4 to 2 mA, so are series resistors set that without weakening the respective beam the amplitudes of the signal voltage of the different LEDs behind the amplifier are approximately the same.

The blue light-emitting diode, a special silicon carbide element, takes with a current of 20 mA without reaching it Signal voltage amplitude and with the unfavorably large spectral half-value bandwidth of 90 nm compared to the other light emitting diodes with a peak width of 25 to 50 nm, a special position.  

The light of the selected LED coincides covered with multiple reflected interference light attached preferably blue-sensitive PIN photodiode, whose short-circuit current is proportional to the light intensity.

The reference voltage is stored in a 10-bit digital memory filed without drift. The memory error +2.5 mV is compared to the high signal voltages for the blank value low and conditional only when using a blue one LED a wrong zero point adjustment (approx +0.005 EE). But this error is most common in measurements negligible and correct by using as actual measured value the difference of the Assumes empty measured values for the respective measured value.

The analog-digital conversion is integrated with an Standard building block performed, the continuous Discharge of the integrating capacitor through a resistor causes a logarithmic conversion and thus display. In addition, unlike usual, the reference voltage comes from the conversion not from a standard voltage source, but of the value stored in the digital memory, so that after a comparison of this circuit part on the display of Value 1g (Uo / U) appears. Its deviation from the real Value is only +/- 0.001 EE over the entire measuring range.

Interference filter because of the single-color light-emitting diodes without a so-called blocking d. H. without an additional Color filters, get by and the half-value range decrease at least 10 nm, are small and easy to face each LED can be mounted.

For the low-pass filter, 100 Hz is sufficient as the cut-off frequency, since due to the full wave rectification the stray light frequency of  mains-powered lamps is doubled. A cut-off frequency of less than 50 Hz is however due to the higher damping prefers.

Due to the special properties mentioned, in particular simple operation and battery supply is that Photometer can be used in a variety of ways. Potential User groups can be found in the area of schools and the private analysis (environmental protection, fish farming, Well control), especially on-site analyzes are to be carried out.

Claims (8)

1. Photometer with a receptacle for a sample cuvette (K), which on the one hand is exposed to the light of a light-emitting diode (PL 1 , PL 2 ...), Which on the other hand weakens the sample cuvette (K) and / or changes its sensor content -Pododiode (PS) is supplied, the photosensor signal is amplified and then supplied analog and / or logarithmic and / or digitally converted to a display device (LCD), characterized in that the light-emitting diode (PL 1 , PL 2 ) by a supply frequency (FS) , which is much higher than usual a stray light frequency, is fed via a current-determining resistor (R 1 , R 2 ) and the photosensor signal is filtered by means of a high-pass filter (BP) with a DC level suppression whose pass frequency range includes the feed frequency (FS), and then one Full-wave rectifier (GL) is supplied with a downstream low-pass filter (TP), whose measured value output signal (x) is displayed. 2. Photometer according to claim 1, characterized in that the light-emitting diode (PL 1 , PL 2 ) is fed according to the feed frequency (FS) by a square wave current with a pulse / pause ratio of 1: 1 and the high-pass filter (BP) is a bandpass filter, whose pass band includes the feed frequency (FS) and several harmonics thereof. 3. Photometer according to claim 1, characterized in that a plurality of light-emitting diodes (PL 1 , PL 2 ), each of which emits light of different wavelength when energized, are arranged on the sample cell (K), and their associated current-determining resistor (R 1 , R 2 ) is so large that the measured value output signal (MS) is the same for each of an empty sample in the cuvette (K) when it is energized. 4. Photometer according to claim 3, characterized in that in the beam path of the light emitting diodes (PL 1 , PL 2 ) a narrow-band interference filter (I 1 , I 2 ) is used. 5. Photometer according to claim 1, characterized in that the supply frequency (SF) is between 400 and 1000 Hz and the cut-off frequency of the low pass below 100 Hz or below 50 Hz. 6. Photometer according to one of the preceding claims, characterized in that at one in the beam path arranged calibration sample the measured value output signal of a digital storage device (SP) is fed controlled and the reference measurement value stored in this way is converted analogously to the Reference input (REF) of an analog-digital converter (ADC) is supplied, the integrating capacitor (CI) via a Discharge resistance (RE) during a conversion process is exponentially discharged, whereby the each converted value is the logarithm of the ratio the reference measurement value for the respective measurement output signal corresponds. 7. Photometer according to one of the preceding claims, characterized in that it contains a control device (ST), which are supplied by a selector switch (WS) for energizing the light-emitting diodes (LS 1 , LS 2 ) control signals which each signal a switchover and in the control device (ST) are stored and output as a flag display signal on the display device (LCD) until a cuvette change is reported by a cuvette sensor contact (T) to the control device (ST), whereupon the measurement value then obtained is digitally converted with respect to a fixed reference voltage and as that Reference value is stored and displayed and that when the cuvette sensor contact (T) is operated again, controlled by the control device (ST), the reference measurement value is converted to the converter (ADC) at the reference input (REF). 8. Photometer according to one of the preceding claims, characterized in that it is battery-powered, has a Zener diode stabilization (Z 1 ) and the reference voltage of the digital-to-analog converter (DAC) is obtained by a further stabilization (Z 2 ).