Saturday, May 14, 2011


  • Microscope 2.5-D Lighting
  • Real RS232 For Laptop/Notebook PCs
  • Mini Lithium Torch
  • In-Vehicle Voltage Regulator
  • Two Keyboards On One PC
Posted: 17 Apr 2011 04:16 AM PDT
Interesting, surprising as well as scientifically useful visual effects can be obtained by employing colored LEDs instead of the normal lamp fitted in the lighting assembly in a microscope base. Of course, the lamp can be simply replaced by a white LED with appropriate changes in the supply current/voltage etc., but it is more interesting to have three colors available — here, red, green and orange (amber) which are individually adjustable for intensity. The dashed line around LEDs D2, D3 and D4 indicates that they are fitted in a 10 mm long section of 1-inch plastic (PVC) pipe.

Microscope 2.5-D Lighting circuit project circuit diagramThe 5-mm holes for the LEDs are drilled at an angle so that the LED beams are pointed downwards at the center of the holder. As shown in the photograph, as small piece of veroboard can be fitted under the ring, with a 1-inch hole in it so that the centre of the ring remains open, allowing the assembly to be placed on top of the glass specimen carrier. The PCB, which is held secured to the ring by the LED wires, allows wires to be run between the lighting assembly and the driver electronics fitted in the microscope base.

Microscope 2.5-D Lighting circuit project circuit diagramThe white LED, D5, takes the place of the (6-V) lamp in the lamp holder, and is connected to the driver circuit by wires. The potentiometers P1, P2 and P3 allow the intensity of each color to be individually adjusted, while P5 acts as the master intensity control. Space allowing, the pots may be fitted in the microscope base. If there are problems with reflections in the ring, paint the inside black. Finally, in most microscopes there is a need for strong lighting which pleads for high-intensity LEDs to be applied in this circuit.

Posted: 17 Apr 2011 04:16 AM PDT
Many Metex DVMs feature a serial interface (RS232) which enables measured values to be copied to a PC for processing. Although this works just fine with most desktop PCs, problems may arise — as the author found out the hard way — when a laptop PC like the IBM Thinkpad 370C is hooked up to the Metex DVM. The cause of the problems is the limited voltage swing of just ±5V on the 370C’s serial interface. This is simply not enough for the Metex DVM, which will appear ‘deaf’ to the laptop.

Real RS232 For Laptop/Notebook PCsReal RS232 For Laptop/Notebook PCsThe voltage swing on the serial interface lines is easily increased with the aid of a dedicated interface driver like the MAX232. With the resultant circuit designed in SMD, it is easily accommodated on a small double-sided PCB that can be fitted a 9-way sub-D adaptor housing. The converter receives its +5V supply voltage from the PS/2/mouse interface (pin 4 = +5V, pin 3 = ground). Current consumption is a modest 4mA or so which has no noticeable negative effect on battery life. The two interface signals TxD and RTS are taken from the laptop to the MAX232 driver inputs, pins 8 and 13.

Real RS232 For Laptop/Notebook PCsInside the MAX232, they are first shaped to proper TTL/CMOS levels and then applied to the actual level converters. The resultant signal then reaches +10 V, which is accepted without problems by the interface inside the Metex DVM. The PCB designed for the converter is small and single-sided. SMDs are the only option when it comes to fitting it all inside the adaptor housing. Do observe the polarity of the electrolytics in this circuit, since an SMD circuit, once built, is difficult to troubleshoot and repair.

Real RS232 For Laptop/Notebook PCsReal RS232 For Laptop/Notebook PCsCapacitors:
  • C1-C4 = 10µF 16V (SMD)
  • IC1 = MAX232-CSE (SMD)
  • K1 = 9-way sub-D socket (female), PCB mount, angled pins
  • K2 = 9-way sub-D plug (male), PCB mount, angled pins, 2 solder pins

Posted: 17 Apr 2011 04:16 AM PDT
This mini pocket torch combines the advantages of Lithium button cells with a super-bright white LED. The lithium cells are small, have a long shelf life and have very little self-discharge. The LED has very high efficiency, an extremely long life expectancy and a modest current consumption (20mA). In combination this results in an exceptionally long-life pocket torch. It is unfortunate that a Lithium cell has a voltage of only 3 V, while a super-LED has a forward voltage of 3.5 V. One cell will not suffice and we therefore will have to connect two in series.

Mini Lithium Torch Circuit DiagramThat results in a power supply voltage of 6 V, and at a current limit of 20mA we need a series resistor of (6 - 3.5) / 0.02 = 125 Ω. Very annoying, because this resistor causes a power loss of 2.5 × 0.02 = 50 mW. Compared to the power consumption of the LED (3.5 × 0.02 = 70 mW) this would mean that nearly 42% of the energy would be wasted! By using an integrated pulse-width modulator from Siemens, the BTS629, this power loss can be limited to about 10%. In combination with two CR2025 cells with a capacity of 170mAh, the pocket torch will have an expected operating time of 15 hours.

Mini Lithium Torch Circuit DiagramTwo CR2032 (230mAh) cells will last an astonishing 21 hours! Another advantage of the IC that has been used here, is that the pulse width, and hence the brightness of the LED, can be smoothly adjusted with P1. The compact printed circuit board for this pocket torch has been designed so that it fits exactly inside the key ring enclosure UM14 from KM. The PCB has a 20 mm diameter hole for the button cells. The negative terminal for the button cells is made with a piece of paperclip soldered to the bottom. For the positive terminal, on the topside of the PCB a flat terminal can be attached with the aid of an M3 bolt and nut, as can be seen in the photograph.

Mini Lithium Torch Circuit DiagramMini Lithium Torch Circuit Diagram

Posted: 17 Apr 2011 04:16 AM PDT
In vehicles it is often required to have a powerful yet stabilized voltage that’s not affected in any way by fluctuations of the battery voltage. The circuit shown here does the job using discrete and inexpensive parts only. While its low cost is a definite advantage over just about any kind of regulator IC, on the downside we have a minimum voltage drop of 2 volts - in fact the output voltage can be set to any value between 1.8 V and about 10 V. Continuous loads up to 100 watts can be handled, while peak values of 140 W should not present problems.

In-Vehicle Voltage Regulator circuit diagramThe power stage consists of two parallel-connected 2N3055 transistors in TO-3 cases. Because of their high base current requirement, a driver transistor type BD241B is incorporated. The feedback voltage arrives at the inverting input of the regulator IC, a type 741 opamp. The level of the reference voltage at the inverting input is adjusted with potentiometer (or preset) P1. The circuit board, of which the layout is given here, accommodates all parts including the two 2N3055 power transistors. As a matter of course, they should be properly cooled.

In-Vehicle Voltage Regulator circuit diagramRemember, the case of a 2N3055 is connected to the collector which is at battery-positive potential. If necessary the voltage regulator may be bypassed by an external switch connecting the battery + terminal with the output terminal. The switch, if used, should be capable of passing considerable currents - at relatively low output voltages (up to about 6 V) currents of up to 15 A (continuous) or 20 A (peak) may be expected. Although the output current is reduced to 10 A when the 10-V level is approached, it is better to be safe than sorry.

In-Vehicle Voltage Regulator circuit diagramIn-Vehicle Voltage Regulator circuit diagramResistors:
  • R1,R2 = 0Ω1, 5W
  • R3 = 1kΩ
  • R4 = 100kΩ
  • P1 = 10kΩ linear potentiometer
  • C1,C2,C3 = 100nF
  • IC1 = 741CN
  • T1 = BD241B
  • T2,T3 = 2N3055 (TO-3 case)
  • K1,K2 = 2-way PCB terminal block, lead pitch 5mm
  • S1 = switch, heavy-duty, 1 change-over contact (see text)

Posted: 17 Apr 2011 04:16 AM PDT
This circuit does the exact opposite of what most of this type of changeover switches attempt to do. Usually a changeover switch is used with two PCs and one keyboard. This version however makes it possible to operate one PC with two keyboards. K1 is connected to the PC for this purpose while K2 and K3 are connected to two keyboards. The data outputs of the keyboards are high in the idle state. As soon as a key is pressed the keyboard will serially transmit the data. The data line will now also be low from time to time. This low level is detected and remembered by the flip-flop circuit around IC1. If the signal originates from K3, the output on pin 6 will be high. Transistor T1 will conduct via resistor R5, which causes the relay to activate.

Two Keyboards On One PC Computer
The signals on K3 are then connected straight through to plug K1. This situation will persist until a signal on the data line of K2 is transmitted. In that case the flip-flop will reverse and pin 6 will be low. The signal at resistor R5 will no longer cause transistor T1 to conduct and the relay will be in the rest position. The signals at connector K2 are now connected to plug K1. The LEDs D1 and D2 indicate which keyboard is connected to the PC. The changeover of the signals via the relay is relatively slow, so the first keystroke is not properly transmitted to the PC. This means that when changing over the first keystroke will always be lost. Also take into account that when the PC is first switched on, the state of the flip-flop is random, so it is not clear which keyboard is initially connected to the PC.

Two Keyboards On One PC Computer

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