New Frost Detector Temperature Sensor Circuit

The choice has again been made for a type that we have already used in many previous Elektor circuits, the LM35CZ (-40 to 110 °C). This sensor is not expensive and generates an output voltage that is proportional to the temperature in degrees Celsius (10 mV/°C).

Sensor
An LM35 is normally powered from a single-ended power supply and 0 °C corresponds to an output voltage of 0 V. It is therefore not possible to measure negative temperatures with an LM35 in the standard application circuit. It is however possible to measure negative temperatures if its output is connected to a negative supply voltage via a resistor. There needs to be a current of 50 μA through this resistor (R2 in the schematic).

We only need to detect the freezing point with this circuit. That is why there is a comparator after the temperature sensor, which turns an LED on if the temperature has dropped below 0 °C during the course of the night. To ensure that the comparator operates properly it is necessary that the measurement value can become slightly more negative with respect to the input. To solve this problem, a diode (D1) has been connected in series with the ground connection of the LM35. The voltage drop across D1 (because of the small current through the LM35 this is only 0.47 V) acts as negative power supply. Since the non-inverting input of comparatorIC2 is connected via R3 to the anode of D1 it functions as the 0°C-reference level for the comparator.

Comparator
The comparator is a standard opamp type TLC271, which we configured for minimal current consumption by
connecting the bias-select input (pin 8) to the power supply voltage. There is no need for the detector to be fast and it will therefore work well with the opamp operating in its most economical mode.

LED D3 provides the frost indication. It is the intention that the LED stays on once the temperature in the room drops below freezing or when it has been below freezing. To realise this, an asymmetric hysteresis is created with the aid of R3, R4 and D2. The instant that the output goes high, the non-inverting input goes more positive via D2 and R4, and the output therefore stays high. The temperature would now have to increase to more than about 30° before the LED will go out by itself. In practice this probably means that it is summer and that it is not likely to freeze anyway. If need be, the hysteresis can be increased by increasing the value of R3.

Capacitor C2 is added to make sure that the LED remains off (the circuit is reset) when the power supply is connected. The non-inverting input of the opamp is briefly connected to ground and the output is therefore low. R1 and S1 are only required if the circuit needs to be reset when the battery is connected. Instead of S1 you could also use a power supply switch or even just simply disconnect the battery for a moment.

Thrifty Power supply
Since the circuit is assumed to be powered from a battery there was a conscious effort to minimise the power consumption. The current consumption of the prototype, at a power supply voltage ranging from 6 to 9 V, was less than 120 μA. When the LED is on, the current consumption rises to only 1 mA at 6V and 1.8 mA at 9V, because a low current LED is used. In our prototype we used a green, low-current LED.

If four AA penlight batteries (with a capacity of about 2 Ah) are used, then the circuit will run for about two years in standby mode. When the LED is on this is considerably shorter, of course (about two months, this is easily long enough to run through a severe winter period). A standard 9-V battery will also last a single winter, provided you frequently check whether the LED is on.

Finally, a comment about the TLC-271CP used here. The version with the C-suffix is specified for an operating range from 0 to 70 °C, but will continue to work at lower temperatures, particularly considering that the IC is not used in a linear application. If in doubt you can always try to get your hands on a version with the I-suffix (that is, TLC271IP: –40 to 125°C). But that is only necessary if you expect it to be real cold in the monitored room...

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New Zener Diode Circuit Diagram

Here we used the 12-0-12 step-down 500mA power transformer. The output of the transformer is supply to the bridge rectifier made of D2 , D3, D4, D5 which is use to convert the Ac supply to the DC supply. Capacitor C1 is used as a filter the DC output. We used  470 μF capacitor  but you can used any. More the value of capacitor more pure DC can be obtained. Resistor R2 of 2.2K is used as bleeder. Here you can see the transistor T1 [BC147B] and transistor T2 [SL100] are use for regulator compressor. The DC output is fed to these transistors. T1 acts as a series pass driver or a current regulator. Base bias for transistor T1 is achieved from the supply through resistor  R3 of 680 ohms  as resistor R2 of  10k is a base bleeder and capacitor C2 1 μF  filters base potential. When the test probe is fully open with no zener connected, the base potential of transistor T1 is around 32V that is across resistor R4 or capacitor C2.

Transistor T1 [BC147B] provides the base potential for transistor T2 [SL100] which acts as a series pass regulator, providing the net DC voltage equivalent to T1 base potential which is fed to the voltmeter.

Now, the voltmeter reads around 30V with no zener diode connected across the probe. When a zener  diode is connected across the test probe, the base potential of transistor T1 falls to zener diode breakdown voltage. With this, the base potentials for transistor T2 and transistor T1 become equal. The meter now shows the actual zener voltage. An adjustment of 0.6 V can be done on the meter scale by shifting the needle with zero adjustment screw on the meter.

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New world Students Share Lessons Uncommon Internship

Its the end of summer and many students are coming out of a brief dip into the professional world  working alongside adults and performing entry level tasks, only to return back to classes in the fall. Some are paid, some are not  some provide both learning opportunities and a chance to build ones network, others less so. Across the board, most students will say that an internship is a better investment of time than sitting at home, or working a regular summer job. 

But some students have found summer experiences that are, arguably, better for their careers — especially those students interested in entrepreneurship, not joining corporate life.

I asked seven students who participated in Lightspeed Venture Partners summer fellowship — a program that behaves like a startup accelerator without taking equity from the teams — how the experience compared to a summer internship, and what knowledge theyll be taking back to school.

Would you consider a non-traditional summer program? Tell us in the comments.

Q&A With Lightspeed Venture Partners Summer Fellows

How did you hear about the program?

I read Y Combinators Hacker News quite a bit. One day when I was browsing, I came across this. We decided to apply and the rest is history. - William Zhou, Planboard

Several of our friends have gone through the Lightspeed program and we heard only good things. - Feross Aboukhadijeh, peerCDN

Through the Stanford Bitcoin Group, a small collection of 8 students with a common interest in Bitcoin. - Matthew Rials, TryBTC

Are you ever too young to start a company?

I think so. At Stanford, various friends were poised to drop out or take time off of school to start a venture. I cant help but think that these folks may have missed out on some of the wonders of American college life. - Giancarlo Daniele, Drive Pulse

Youre never too young to start a company. I started my first company when I was in Grade 10 in high school. My former Co-Founder, David Kim, and I started a web design firm. Slowly, our client base grew, but we were tired of all of their rules. We decided to scratch our own itch. We ended up launching Draftboard for the designer community [2] in Grade 12. That just shows you how young we were in our ventures. - William Zhou, Planboard

Will you continue with your project after the summer is over?

We certainly plan to carry on our project as we transition back to Stanford, once summer has ended. We are very passionate about the notion of spreading Bitcoin to the world and easing the average users transition into the new currency, and we will continue to work on TryBTC for as long as that takes. We have already raised enough money to keep things running through school, and our investment terms allow for us to remain enrolled in classes. - Matthew Rials, TryBTC

Absolutely. During this summer we’ve had amazing traction with Contastic – our mobile CRM product. The team has grown from 1 person (just me) to six during the 10 weeks. Together, we’ve been able to build a complete product and land pilots at several fortune 500 companies. With the support of Lightspeed and our early customers we’ve been able to develop a truly unique product that will improve the lives of salespeople worldwide. - Cy Khormaee, Contastic

Why did you choose this program over a regular internship?

There are few more rewarding ways to spend working hours than to directly think up and build products and wait to see how people use them. You cant usually do this at a traditional internship. - Giancarlo Daniele, Drive Pulse

Theres nothing in the world like working for yourself — no pointy haired boss, no managers — just complete control over how you spend your time and what you make of your life. Starting a company is a high-stress, high-risk decision, but its also high in learning, impact, and fun. We dont wish that we were doing anything else. - Feross Aboukhadijeh, peerCDN

What did you learn over the course of the summer?

While its true that I became a better programmer, the majority of what I learned this summer would fall into the "professional development" category. There are an astonishing number of soft skills that are absolutely crucial to managing a company, and the people that work for it. 

For instance, taking a product from prototype to production involves managing relationships with hardware designers and manufacturers, electronics designers and manufacturers, packaging designers, retailers, your own internal sales team, your product development team of engineers, your lawyers, your investors, and co-founders. If for example, you didnt know what to look for in a designer when you interviewed your first, you certainly get a better sense by the time you speak to the tenth. Lastly, the most empowering takeaway from the summer is the following: 

I validated that being an entrepreneur is what I was born to do. Whether in this venture or another I have equipped myself with the skills I need to be a successful visionary and leader, and am incredibly grateful for the opportunity Lightspeed gave me to do so. - Aaron Konigsberg, Lapshark via

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